Abstract: A padlock includes a casing having a bottom shell and a cover shell pivoted with each other, a shackle movably mounted in the bottom shell, a constraint member movably mounted in the cover shell and having a hook portion for hooking the shackle to lock the cover shell and the bottom shell, and a combination lock unit mounted in the cover shell for controlling the constraint member to determine whether the constraint member is allowable to move away from the shackle.