A utility knife includes a knife holder slidably received in the front portion of two casings. The front portions of the casings are hooked together. A knob is pivotally coupled to one of the casings and includes one end biased to engage with the other casing. A spring is engaged between the rear portions of the casings for biasing the rear portions of the casings away from each other when the knob is disengaged from the other casing and for allowing the casings to be easily separated from each other by the user.