A bobby pin includes a left body and a right body respectively having plural ribs that are formed with plural teeth for engaging with each other to keep hair firmly clamped. The left and the right body have an opening device that is provided with a root member respectively, with a pivotal pin mounted with a compression spring and a torsion spring. In using, press inward the two bodies to keep the teeth engaged with each other for clamping hair. And, as long as the opening device is pressed, the two bodies can be immediately separated from each other for releasing hair.