Citing two NBC executives knowledgeable of the situation, the newspaper said NBC Entertainment Chairman Robert Greenblatt took offense to Leno's Feb. 28 "Tonight Show" monologue, poking fun at the network's expense.
Leno joked about reports NBC fell into fifth place behind Spanish-language network Univision in the sweeps month of February.
"For the first time in history NBC is going to finish fifth in the ratings period," Leno said during the monologue. "We are behind the Spanish-language network Univision -- or as we call it here in Los Angeles: Cinco de Ratings.
"It's so bad, 'The Biggest Loser' isn't just a TV show anymore; it's our new motto," Leno said.
Greenblatt sent an email to Leno complaining about the jokes, the Times reported.
Leno was taken by surprise by Greenblatt's complaint and defended himself, saying late-night comedians poking fun at their own networks is a long tradition in late-night talk shows, the sources said.
NBC recently denied trying to force Leno out of his job as host of "The Tonight Show" in favor of Jimmy Fallon.