A folding chair having seat, backrest, front leg, and rear leg interconnected with backrest pivoted to front leg and back leg, and seat pivoted to front leg and back leg so as to lock backrest when chair is unfolded, and when weight is applied to the seat a force is applied further locking the backrest. A modification provides a folding chair with outer and inner frames for locking backrest and rear leg in position when chair is unfolded for use.