Recently, there are some applications of NMR imaging to in-vivo microscopy because it introduces the chemical properties of microscopic objects. But it suffers from low SNR, long imaging time and especially the diffusion effect. The diffusion effect on NMR microscopy increases as the spatial resolution is improved. The diffusion affects the signal decay when the field gradient is applied, thereby degrading the spatial resolution along the reading and coding gradient directions. This diffusion effect is discussed in detail. The optimal data acquisition time and the field strength of reading gradient are analyzed in order to maximize the signal to noise ratio for a given resolution and the new imaging scheme is proposed for reducing the diffusion effect.