Abstract: If photon is massive, it will have a minimum frequency and the velocity of light will not be a constant but have a low limit about 10 % lower than constant c. Different from the dispersion of the velocity of light in medium, the dispersion in vacuum caused by the photon's mass concentrically appears within a narrow frequency region nearby minimum frequency and，so that, as the frequency is higher than that of long wave radio, the dispersion becomes zero and the velocity of light should be saturated (i. e. equals to constant c). In the result, the velocity of light equals to constant c of well known and Compton scattering should not necessarily mean that the photon is massless. It is tried to demonstrate that propagation velocity of the static field should be equal to the low limit of the velocity of light, and the latter means that there is the dispersion. Therefore, experimental determination of the propagation velocity of static field equates with observation of dispersion and is helpful to answer whether the photon is massive or massless.