A mainframe structure includes a housing having a front opening, flanges on one same plane around the front opening and stop members suspending on the inside corresponding to the flanges, a circuit module accommodated in the housing, and a cover detachably mounted in the front opening of the housing and stopped against the flanges and the stop member. The cover has two slots symmetrically disposed at two opposite lateral sides, two lugs respectively disposed adjacent to the slots and two handles respectively mounted in the slots and pivoted to the lugs in reversed directions. By means of biasing the two handles to stop against respective stop members, the user can detach the cover from the housing conveniently with the hands without any hand tools.