A foldable knife comprises a blade pivotally connected to a pivotal portion at a first end of a handle. A resilient bar extends between a second end of the handle and the pivotal portion. When the blade is received in the handle, the resilient bar biases the blade with a torque to rotate the blade into the handle. When the blade is rotated outward from the handle up to a predetermined angle, the resilient bar biases the blade with a torque to rotate the blade out of the handle. A switch element has a first end extended out of the handle for being switched. When the switch element is switched, a second end thereof biases the blade and rotates it out of the handle. The user can use only one hand to hold the foldable knife and switch the switch element to make the blade spring out of the handle.