Icelandic Airlines, a long-time low-fare airline, will begin new scheduled service to the European continent from Baltimore-Washington International Airport this fall for $155 each way.

Beginning Nov. 4, Icelandic will fly a McDonnell-Douglas Super DC8, which seats 249, to Luxembourg from BWI every Saturday evening. Although the return flights from Luxembourg to BWI will depart on Fridays, passengers who want to return other days can fly Icelandic to New York and then transfer to another carrier for passage to BWI at no additional charge.

Assuming Civil Aeronautics Board approval of the proposed fares, passengers will be able to buy a one-way ticket to Europe for $155 or a round-trip ticket for $310. Icelandic said it will be advising travelers to buy a round-trip ticket before departing the U.S. becaused return tickets purchased in Europe would cost more. Icelandic will refund the unused portion of a round-trip should a traveler elect to return on another airline.

The flights will take about 8 1/2 hours, plus a 45-minute stop in Reykjavik, Iceland, about the same time it takes to get from here to other European cities in the area. In Europe, Icelandic uses the airport in the City of Luxembourg, the capital of a grand duchy with the same name which is bordered by France. Germany and Belguim.

Peter Larsen, Icelandic's district sales manager here, said yesterday that between 15,000 and 20,000 Icelandic passengers a year use the stop-over opportunity to stay in Iceland for a day or more on their trips abroad.

The single-plane, single-class Icelandic service to Europe has no advance-purchase or minimum or maximum-stay requirements and no meal charge.

Icelandic has been offering low fares to Europe for some time from New York and Chicago but never has used its authority to originate flights from Washington.

"We conclude that there's tremendous potential for growth here in the low-cost, translantic air business," said John J. Loughery, Icelandic's vice president and director of its Western Hemisphere operations. "We intend to fill the void that's existed."

He said the airline chose an autumn startup to gauge consumer reaction during a normally slower travel season and that will increase service if need be.

In another low-fare development, Eastern Airlines has asked the CAB for approval of a new fare plan between BWI and San Juan, Puerto Rico, that will offer travelers a 35 percent discount from current coach fare levels.

If approved, beginning Oct. 2 Eastern would offer seats on flights south-bound between Friday and Sunday for $99 one way compared with the current $136 weekend coach fare. On flights northbound Monday through Thursday, the fare would be $89 one way compared with the current $130 midweek coach fare.

In order to tak advantage of the new fare, travelers would have to buy their tickets 14 days in advance and the number of seats per flight available would be limited.