The near field distribution of uncoated and metal coated fiber optic probes were characterized by the method of three dimensional finite difference time domain (FDTD). The result reveals that, for an uncoated probe, propagating wave emitting from the conic side dominates the near field distribution, while a film of metal is coated, the coated probe can confine the wave more strictly inside it. Second, the polarized incident wave becomes depolarized after emitting from the small aperture of two kinds of probes. In other words, there are two electric field components whose polarization direction is perpendicular to that of the incident one. Accordingly, the image contrast can be improved if the polarized components instead of the total field are detected. The relationship between the distribution characteristics of near field distribution and the phenomenon of depolarization is analyzed in detail.