Abstract: The formula of the in-plane forced vibration of piezoelectric materials plates is presented based upon the fundamental piezoelectric equations, then a direct method of measuring the piezoelectric constant of piezoelectric materials is developed by combining the response formula of the in-plane forced vibration of piezoelectric materials plates with the resonance test. The resonance test is carried out by using laser vibratormeter that has high measurement accuracy and do not need sensors, made the test more convenient. Comparing with the traditional method of measuring the piezoelectric constant, the present method has much more advantages. The accuracy of the measuring results is also raised due to the reduction of intermediate links.