A thickness extensional piezoelectric resonator includes a substantially rectangular piezoelectric strip, first and second excitation electrodes disposed on both surfaces of the piezoelectric strip, and an internal electrode disposed inside the piezoelectric strip. The first and second excitation electrodes are located on opposite sides of the piezoelectric strip. The internal electrode is located opposite to the first and second excitation electrodes. Assuming that the piezoelectric plate has a width W and a thickness t, the piezoelectric plate is constructed such that W/d is less than about 6.1 if d=t/n.