A utility knife including a housing (19), two parallel knife holders slidingly disposed substantially parallel to each other inside the front section of the housing; different blades (5) are positioned on the two knife holders; each push button (6) is movably disposed on top of one knife holder (3); a strip-shaped aperture (17) is provided on top of the inner wall of the housing (19); the strip-shaped aperture has two grooves (1e), parallel to each other, for receiving the two push buttons and for allowing the two push buttons (6) to slide along the two parallel grooves; and a plurality of recesses (1a) are distributed inside each groove; each push button further comprises a snap block (61), when the snap block (61) is inserted into the corresponding recess (1a), the corresponding knife holder (3) and the blade (5) are locked in position.