Bertha E. Cohen 67, Cambridge property-owner familiar to many Harvard students living off-campus, died Monday in her Memorial Drive apartment. She leaves a fortune estimated at over $20 million.

The owner of many buildings that the University has long wanted to buy, Miss Cohen once said she delighted in being known "as the thorn in the side of Harvard."

She came to this country as a young Polish immigrant with little money, and began work as a milliner in the old Chandler Co. department store in Boston. Saving from her weekly salary, she soon launched the real estate career which left her, as many friends used to say, "practically owning Harvard Square."

Throughout her long years in business, Miss Cohen always maintained complete control of her widespread interests. She always paid close personal attention to the minute details in her transactions.

She headed nine realty and investment firms in the Greater Boston area and owned numerous complete apartment and business blocks in Boston, Cambridge, Melrose, Malden, Dorchester and Roxbury.

Her companies included Curtis Realty, Inc., Cober Realty Inc. Hemenway Investment Co., Melrose Realty Co. Inc., and the Strathcona Realty Trust, which maintained offices in her apartment building on Memorial Drive.