Donald I. Baker, assistant attorney general for antitrust, said yesterday the division would recommend 18-month prison sentences for individuals convicted of felonies under the Sherman Act.

The 18-month period will be a "base" sentence recommendation that will be raised - to the maximum three years - or lowered depending on aggravating or mitigating factors, Baker said in a memorandum on guidelines for sentencing recommendations to division attorneys and economists yesterday.

Increasingly, cases filed by the antitrust division will involve violations continuing or occurring beyond Dec. 1974, making individuals involved in them subject to the stiffer penalities under a law making price-fixing and other hard-core violations of the Sherman Act felonies, instead of misdemeanors, and punishable by up to three years in jail and fines of up to $100,000 for individuals and up to $1 million for corporations.

"A primary purpose of these guidelines is to provide a means by which sentencing recommendations will be more rational and equitable," Baker said yesterday.

Factors which could raise the jail term recommended to the courts are the size of the potential impact on the economy, the "leadership" position of the individual in the crime, the extent of threats, force, or economic reprisals used to further the conspiracy, its lenght of time, and any previous conviction. Factors which could lower the jail term from the "base" are cooperation with the government and personal family or business hardships.

Baker said a prison term is the division's preference, but in some cases fines will be appropriate. As a practical matter, too, Baker suggested, the division would probably have to back up their prison recommendations with a recommended fine in case the court should reject all or part of the prison suggestion.