Two Bethesda employers with a total of more than 300 people on their payrolls plan to move their headquarters from Montgomery County to Frederick, 40 miles north of Washington.

The Aircraft Owners and Pilots Association and Avemco Corp., an airplane insurance and finance company with links to the AOPA, plan to put up new headquarters buildings at the Frederick airport, executives said yesterday.

Both now are based in the Air Rights Building at 7315 Wisconsin Ave.

Avemco President William P. Condon said his firm has obtained a $3.5 million tax-free industrial revenue bond from Frederick County to finance its new home office. Construction of the 40,000-square-foot office for Avemco's 100 employes will start this spring, and the firm plans to move by June 1982.

The AOPA, which has about 200 employes, also is seeking tax-exempt financing for new headquarters facilities and hopes to move within two years, said Charles Spence, vice president for public affairs. The civil aviation group will open a small congressional relations office in downtown Washington when it moves the majority of its staff out of the city, he said.

Spokesmen for the insurance company and the trade association said they are moving to Frederick to avoid the high cost of facilities in or near the District of Columbia and to get an airplane location.

"We have always wanted to be at an airport," said Avemco's Robert Roe. "It's both an image and a convenience. All our other seven offices are at airports." Avemco insures and finances private aircraft, and for many years has had a contract with the AOPA to provide services to its members.

"It'll be much cheaper for us to have our own building than to pay rent," added Roe.

Spence said AOPA officials "look for it to be a whole lot less expensive [to move to Frederick] than to be right in the middle of Bethesda. . . . The airport location is important to give our members an opportunity to fly to the home office."

The AOPA's Air Safety Foundation flight-tests aircraft routinely, Spence said, and both the AOPA and Avemco use light planes extensively for business travel.

Spence said the AOPA expects to spend about $5 million for an office building and a separate printing plant, and like Avemco is seeking financial aid from Frederick officials.

The Frederick County supervisors yesterday sold the $3.5 million worth of bonds for Avemco's building to Union Trust Co. of Maryland and will pass the funds on the Avemco.