Prince George's County Councilman Francis B. Francois, who is considered one of the county's preemiment Democratic politicians and has held elective office for 18 years, is planning to leave his council post to become executive director of a transportation lobbying group, sources said yesterday.

Francois, 46, nationally known as an expert on transportation, local government and urban affairs, is expected to make the annoucement today. He would not comment last night on reports by sources that he plans to join the American Association of State Highway and Transportation Officers at an annual salary in the mid-$50,000 range.

Such a move would be certain to initiate a period of intense maneuvering among county Democratic leaders to select his replacement. It also is likely to generate increased speculation about the Democrats' nominee for county executive in 1982.

Francois has been considered by some observers the most qualified Democratic nominee for executive and a likely opponent to Republican Executive Lawrence J. Hogan, should Hogan seek reelection.

It could not be learned how soon Francois, a patent attorney who lives in Bowie, plans to leave his part-time at-large council post, which pays $24,000 a year.

He served on the 11-member council since 1971; before that he spent five years on the Board of County Commissioners, the county's previous governing body, after serving for four years on the Prince George's Orphans [proate] Court.

In addition to his service with the county government, Francois is president of the National Association of Counties and serves on the Metropolitan Washington Council of Governments and the Washington Metropolitan Area Transit Authority. He would have to give up these positions on leaving the county government.

The organization that Francois will direct was established in 1914 and lobbies both Congress and the White House on transportation issues. Its members include representatives of transportation and highway agencies of the 50 states, the U.S. Transportation Department and some transportation agencies in Canada. Its aims include both developing national transporation goals and worlwide engineering and technical standards in the field.

Francois' successor on the council would be chosen by a majority of the council wihtin 30 days of his resignation. Because he is a Democrat, the county's Democratic Central Committee will be responsible for submitting a list of three names, from which the all-Democratic council can make its choice.

Because there is a long-cultivated network of ties within the county's Democratic party, a successor is likely to have been selected well before the central committee submits its list.

Should the council be unable to agree on a replacement within 30 days, the choice would be made by Republican Hogan, who would be bound to choose from those on the Democratic committee's list.

A successor is likely to come from the Bowie area, as Francois does, and to have strong ties to the party's dominant wing, which has been in disarray since the 1978 elections but currently is trying to regain cohesion in preparation for the 1982 elections.