A foldable knife includes a handle, a frame pivotally connected to the handle, a cartridge pivotally connected to the frame, a leaf spring attached to the frame, a blade-driving device, a blade-positioning element and a button. The frame includes a slot. The cartridge includes a slot through which the leaf spring reaches at least one blade when the cartridge is disposed in the frame. The blade-driving device includes a plate and a switch. The plate is in firm contact with the blade. The switch includes a portion located outside the frame and another portion inserted through the slot of the frame and connected to the plate. A blade-positioning element is movably connected to the frame. A button includes a portion for pushing the blade-positioning element and another portion inserted through the aperture of the frame for contact with a user's finger.