Prince George's County Executive Lawrence J. Hogan has decided to appoint a longtime friend with expertise in the field of economic development to head the county's Department of Program Planning and Economic Development.

Hogan is expected to officially announce this week the appointment of John W. Cawthorne, who is now the executive director of the National Association of State Development Agencies.

Cawthorne has been on an $18,000 retainer with the county since last June to help the county attractt international firms and set up a foreign trade zone in Prince George's.

He will take over as head of the economic development department in April provided his appointment is approved by the County Council.

The department head position has been vacant since last week when Paul Gilbert unexpectedly resigned the post. Gilbert, who was appointed after Hogan was elected county executive in 1978, apparently resigned after learning that Hogan was dissatisfied with his performance.

Gilbert's relations with Hogan apparently were strained since this summer, when a series of reports appeared questioning possible conflicts of interest in Hogan's support for some loans concerning economic development in the county.

Gilbert has been unavailable for comment. Hogan aides said that Gilbert resigned for personal reasons.

Cawthorne has been executive director of the National Association of State Development Agencies for the last 11 years.