Fashion-oriented department store retailing arrived in Frederick yesterday under a relatively unknown name here that may become more prominent in the next decades.

Hess's of Allentown, Pa., one of the country's most unusual and innovative department stores, formally opened a 100,000-square-foot unit in the Francis Scott Key mall outside Frederick. The opening was greeted by local civic leaders and thousands of curious customers who mobbed the store.

Hess's joins Sears, Roebuck & Co. as major tenants of the shopping center, signaling the arrival of more-affluent-oriented retailers to Frederick County, which is becoming more of a refuge for workers in the Baltimore and Washington areas who cannot afford housing close to those cities.

For Hess's the expansion to Frederick also is important. The Maryland store is the most distant from the company's Allentown headquarters in Northeast Pennsylvania and could be a vehicle to begin wider branching in Maryland and Virginia.

Irwin Greenberg, president of Hess's said in an interview yesterday that, if the initial success in Frederick continues, he expects up to six stores in Maryland. After that, Hess's could expand across the Potomac into Virginia, he added.

A full-range department store.Hess's offers a traditional mix of medium-priced to higher-priced furniture, apparel and other goods in addition to specialties that have been a hallmark since the first Hess's store opened in 1897. The company was the first to sell topless swimsuits and the past two weeks was the first retailer in the world to offer special "acquapressure" rings for diet control.

As shoppers walked through the store yesterday, some said it was comparable to Bloomingdale's and Neiman-Marcus. Indeed the store's initial sale was $500 worth of silver - indicating the growing affluence of the Frederick area. Wall Street apparently liked what it heard from the Maryland community: Hess's stock on the American Stock Exchange hit a 1978 high of $14 at yesterday's close.

Hess's Fredserick store features 15,000 square feet of furniture and is emphasizing fashionable home furnishings because of a home building boom in the area. There is a large gift department, a big gourmet food and candy section, the biggest cosmetics department Hess's has opened, and what is reputed to be one of the largest U.S. retail selections of jade, art and other merchandise from China.

The Frederick store is No. 16 for Hess's which until now has concentrated iits business in the northeast section of Pennsylvania. The chain will open its 17th store in Harrisburg this fall and plans a store for Utica, N.Y.

Philip Berman, Hess's chairman, said yesterday he intends his store to be "the hometown department store for Frederick County," which has 100,000 residents. "With quality shopping here, new and available products, styles and fashions, they won't have to drive to Baltimore and Washington, he said.

Berman, who has headed the firm since 1968, said Hess's has buyers in California, Chicago and New York and representatives in England, France, Germany and Italy seeking out unusual merchandise.

The Frederick store employs about 180 persons, virtually all of them locally hired.

In the year ended Jan. 31, Hess's sales volume was $120 million, and Berman has forecast that five new stores will add $25 million of sales in the next year. Hess earned $3.05 million ($2.36 a share) last year compare dwith $2.,96 million (2.25) in 1976.