The mathmatic model of light emitted from optic fiber as source is given, and physical characteristics of the light is discussed. The relationsghip between V number of optic fiber,and coherent length & mean intensity extension of light emitted from fiber with a Gaussian intensity distribution is obtained in the Franhauf diffraction domain. The comparasion between this theory and diffraction field of incoherent source agrees with Van Cittert-Zernike law is made. The experimental result conincides with theory.