A utility knife is provided, including: a housing, including first and second compartments; a blade holder, movably received within the first compartment; a blade storing carrier, detachably inserted in the second compartment, including a receiving portion for receiving at least one spare blade; an elastic unit, disposed between the housing and the blade storing carrier; and a locking mechanism, including a base body disposed to the housing, a movable member attached to the base body and movable between locking and release positions, a first engaging member disposed on the movable member and a second engaging member disposed on the receiving portion and releasably engagable with the first engaging member, when the is second engaging member is disengaged from the second engaging member, the elastic unit biases the receiving portion to eject the blade storing carrier.