A window blind includes a headrail, a bottom rail, a plurality of slats, and a cord. The slats are arranged between the headrail and the bottom rail, and each slat has two bores and one or plural of locking slots. Each locking slot has an open end and a closed end, and the opening end of the locking slot is connected to the bore. The cord passes through the headrail and the bores of the slats, and being fastened to the bottom rail. A diameter of the cord is greater than or equal to a width of the locking slot. As a result, the cord goes into the locking slot quickly and can be clamped by edges of the locking slot while a kid is trying to pull the cord out of the slats.