The chief of Mexico's state oil company, Jorge Diaz Serano, unexpectedly resigned tonight because of a dispute with other Cabinet members over this week's sharp drop in Mexican oil prices.

Diaz Serrano, whose post gave him Cabinet status, had been director general of the company, Pemex, since December 1976. He was immediately replaced by Julio Moctezuma Cid, a former finance minister and until tonight coordinator of national development projects. The change, while it could affect the recent price drop, was not expected to alter Mexico's overall oil policies.

The resignation came on the eve of a visit to Washington by Mexico's President Jose Lopez Portillo.

Diaz Serrano, a former private oil contractor and a friend of Lopez Portillo, expressed regret in his letter of resignation that "my decision to reduce the price of crude oil did not receive the unanimous approval of the economic Cabinet." But he did not discuss the arguments for and against his decision.

On Wednesday, Pemex announced it was cutting the price of the mix of light and heavy crude that makes up most of its exports from $34.60 to $30.60 a barrel for at least a month. This means a daily loss of income for the government -- which has an oil monopoly -- of about $5.6 million a day.