Ignacio Novo Sampol pleaded guilty yesterday in U.S. District Court to one count of making a false statement in October 1976 to a federal grand jury that was investigating the car-bombing assassination of former Chilean ambassador Orlando Letelier.

Novo, 44, had been convicted by a jury in 1979 of lying to the grand jury and concealing information about the crime. From the time of his arrest, Novo had served a total of more than two years in prison until a federal appeals court reversed his convictions in September 1980.

Judge Barrington D. Parker yesterday said that Novo's sentence in connection with the guilty plea would be covered by the time he had already spent in jail. The penalty for making a false statement to a grand jury is five years in jail, a $10,000 fine or both. In exchange for Novo's guilty plea, the government dismissed the remaining charges against him.

The U.S. Court of Appeals ordered a new trial for Novo after ruling that his case should have been heard separately from more serious murder and conspiracy charges against his brother Guillermo and Alvin Ross Diaz. Those two men, who like Ignacio Novo are staunch anti-Castro Cubans, were convicted of killing Letelier and a young associate, Ronni Karpen Moffitt, when a remote control bomb exloded under their car as it rounded Sheridan Circle along Embassy Row on Sept. 21, 1976. Ignacio Novo was not charged in the killings.

In that same appeals court decision, Guillermo Novo and Ross were also granted new trials because the court said Parker had improperly allowed the jury to hear evidence against them by two jailhouse informants. Guillermo Novo and Ross, who had been serving life terms in prison, were then acquitted by a second jury of the conspiracy and murder charges at a retrial last spring.

Letelier, 44, had been a vocal critic of Chilean dictator Augusto Pinochet. U.S. government prosecutors had contended that the Cubans agreed to carry out the murder, with directions from star prosecution witness Michael Vernon Townley, an American-born agent for the Chilean secret police, to win favor with the Pinochet government. The Cubans argued they were made scapegoats by the Chileans who wanted to shield the Pinochet government from blame and salvage diplomatic relations with the United States.

Guillermo Novo, who was convicted after the second trial of lying to the Letelier grand jury, was sentenced last June to 4 1/2 years in prison, but will have to serve no more than nine months in jail based on the terms of his sentence.

Chile has refused to extradite three military men, including the head of the secret police, known as DINA, who were indicted in connection with the Letelier murders. Law enforcement officials are still searching for two other Cubans who have been charged in the case.