The wide distribution of the energy unit of a photon hνwith regard to infrared，visible light， ultraviolet，Gamma rays and X rays suggests that photons are not the elementary particles，and each of the photons consists of N micro-photons［１］of egual mass．We find that the product of the wavelengthλof photons and N（λ）is an invariantζ＝N（λ）λ［２］．Then a new interpretation of Plack’s constant and wave-particle duality was formed based on the Micro-photon theory．That the wavelengthλof photons is but the distance interval between two adjacent Micro-photons and frequencyνrepresents the number of Micro-photons emitted per unit time．Guided by spatial structure of N number Micro-photons，the present paper in addition deduces it is invariably true that c＝λνdefincted in all inertial frames is an invarient．Now，the Principle of of light Velocity invariant by Einstein is no longer a hypothesis，but a fact．However，the survey results of the wavelength and frequency differ sometimes in different inertial frames，which will probably challenge the Principle of Relativity．In addition the formulas of aperture difraction，refraction and reflectron conducted．