The Nevada Athletic Commission investigated McGregor, 28, and Diaz, 31, after they threw bottles at each other before their Ultimate Fighting Championship rematch in August.
The Irishman went on to win the UFC 202 fight on points.
That followed American Diaz's victory over McGregor at UFC 196 in March.
McGregor tweeted: "I get fined more than these bums get paid."
Diaz's case will be heard at a later date.
Conor McGregor has been given 50 hours of community service and fined $150,000 (£121,000) following a confrontation with Nate Diaz in a press conference.