A new interpretation of Planck's constant and wave-particle duality was formed based on the micro-photon theory. From experimental data on hydrogen spectral lines, it is found there is an invariant ζ= Nλ which suggests that an energy unit of a photon hf contains N micro-photons of equal mass,whereλis the distance interval between two adjacent micro-photons and f represents the number of micro-photons emitted per unit time.By using micro-photon theory to calculate the velocity of light,it is found that c=fλ is really an invariant constant with whatever inertial reference frame.The conclusion is that the principle of light velocity being invariant by Einstlein is not a hypothesis, but really a fact.