Miami leapfrogged the Celtics with the win, their first over Boston in four attempts this season.
Big Three take Heat to second place in Eastern Conference
The Miami Heat have taken control of second place in the Eastern Conference after a 100-77 rout of their nearest rivals Boston. James scored 34 points, grabbed 10 rebounds and had seven assists for the Heat.
Miami leapfrogged the Celtics with the win, their first over Boston in four attempts this season. Paul Pierce led Boston with 24 points.
With Boston losing 95-94 against Washington, Miami's (57-24) win set up a postseason match-up with the No 7 seed Philadelphia.
"This game was just as important as yesterday's, because it was drawing closer to wrapping up the two seed," Heat guard Dwyane Wade told reporters.
Wade scored 21 points, and Miami also got 15 points and nine rebounds from Chris Bosh, the final member of the team's 'Big Three'.
The Los Angeles Lakers' worrying slide to end the regular-season continued as they went down 120-106 to the Oklahoma City Thunder, their fifth straight loss.
Kevin Durant scored 31 points and Russell Westbrook added 26 as Oklahoma City climbed to within a game of the Lakers in the battle for the No. 2 seed in the West.
The Dallas Mavericks are now also one game behind Los Angeles after beating the Phoenix Suns 115-90.
Dirk Nowitzki had 19 points and Shawn Marion added 18 and 11 rebounds.
Chicago have already secured top seed in the East and enjoyed yet another win as they beat the Orlando Magic 102-99 behind 39 points from Derrick Rose.
Orlando thought they had tied it with a Jameer Nelson three at the buzzer, but it was called no basket.
The Memphis Grizzlies continued their push for a high seed in the West by beating the New Orleans Hornets 111-89 behind 18 points from OJ Mayo and 16 more from Marc Gasol.
Carmelo Anthony poured in 34 points, scoring the winning basket with five seconds left, as the New York Knicks held off the Indiana Pacers 110-109.
Rodney Stuckey's 24 points led the Detroit Pistons to a 112-101 win over the Charlotte Bobcats, ending a run of 11 road games without a win.
Jerryd Bayless had 19 points as the Toronto Raptors beat the New Jersey Nets 99-92, while Marcus Thornton had 21 points to power a come-from-behind win for the Sacramento Kings, who topped the Golden State Warriors 104-103.
Ex-Hornets owner talks of his attempts to keep the team in New Orleans
George Shinn said he sold the Hornets for about $50 million less than he could have because he wants the club to remain in New Orleans, and he would be willing to buy back in as a minority investor should a viable Louisiana ownership group come together.
Shinn, who has rarely been to New Orleans since selling the club to the NBA in early December, returned Monday to present $500,000 from his foundation to a Salvation Army program aimed at helping the working poor find stable housing.
Afterward, Shinn said he plans to donate much of his wealth to charity, and for the same reason, rejected a $350 million offer from Oracle founder and CEO Larry Ellison.
"He sent me an offer in writing and I just couldn't find it in my heart to do it," Shinn said, adding that he worried Ellison would move the club to the West Coast.
Moore is top pick in WNBA draft
Maya Moore was selected by the Minnesota Lynx with the first pick in the WNBA Draft on Monday.
Moore helped UConn win a record 90-straight games and also led the school to two national championships.
The four-time All-American averaged 22.8 points this season and 19.6 during her career. She was also the only collegian to play on the US women's national team that won the gold medal at the world championship last October.
Moore is the second straight UConn player to be taken with the first pick in the draft. Tina Charles went to the Connecticut Sun last season and earned the league's rookie of the year honors.
Moore was followed in the draft by: Elizabeth Cambage of Australia (Tulsa), Courtney Vandersloot of Gonzaga (Chicago), Amber Harris of Xavier (Minnesota) and Jantel Lavender of Ohio State (Los Angeles).