We present three-dimensional simulations of the image formation of microstructure in near-field optical microscopy with the three-dimensional finite-difference time-domain method (FDTD). First, we calculated the intensity distributions inside and outside and the flux densities for both the tapered and parabolic fiber probes used in near-field optical microscope and nanolithography. The calculating result shows that for different kinds of shape the intensity distributions in both probes are similar and present standing wave forms; but the amplitudes and locations of peaks of the standing waves are different from each other. The intensity outward parabolic probe is higher than that outward tapered probe. Then we computed the intensity distributions of the samples which are composed of different materials by different polarization illumination. Assuming an aperture-type probe in collection-mode near-field microscope, we compare the images produced from the sample composed of three dielectric blocks in nanometer at a distance of 25nm and 150nm, respectively, under constant-high-scanning mode along with the direction of the polarization of the illuminating light, with near-field distribution of the sample without probe. The results show that the probe disturbs the original field distribution of the sample. The received signal is different from the original field distribution of the sample. However the received signal contains high frequency information of the sample in near-field region. Due to probe-sample interaction, parts of evanescent field transform into propagation wave. Only the interaction between the probe and sample in the near field makes possible to probe the high-frequency components and achieve the super-resolution. Therefore, the detected resolution depends on an assembly of the tip size, shape of the tip, distance between tip and sample, relative position and material characteristics of both tip and sample. These results provide the basis for correct interpretation of experimental work.