Abstract: The classical experiment of equivalent of gravitational mass and inertial mass are reviewed. The discovery of the principle of equivalence is introduced. The application of the principle of equivalence to the delay of time of the gravitational field, gravitational red of spectrum of star and deflection of light ray are illustrated.

Key words: gravitational mass, inertial mass, principle of equivalence, reference system of part, delay of gravitational red shift, deflection of light ray