Proximity printing is one of the lithographic processes used in fabricating semiconductor, micro optoelectronic components and micro electro-mechanical systems to transfer mask pattern to the substrate. This research investigates the cause of pattern distortion based on the analysis of diffraction effect of light beam due to the gap between the mask and the photo resistant in the proximity printing process. The Huygens-Fresnel diffraction theorem is first employed to derive the diffraction effect. It shows that irradiance decreases as the gap increased, which is consistent with practice. Further investigation shows that shading area can be exposed, even with a small gap, due to the diffraction effect that makes the exposed image imprecise. A deeper analysis on the diffraction effect in microlithography of proximity printing with single-aperture, dual-aperture, and multi-aperture masks is investigated. It shows that the contrast irradiance between the exposed and the shading regions becomes closer as the gap increases. This makes the boundary of image become vague and causes the exposed image to be distorted. Experiments are then designed and conducted to verify the derived results. Experimental results using φ80μm circular single-, dual- and multiaperture masks showed that the boundary of adjacent circular apertures begins to crossover with each other when the gap reaches 240μm. When the gap increases to 360μm, interference between adjacent circular apertures becomes stronger that results in hexagonal images. This shows that experimental results are consistent with simulations from theoretical analyses. The main contribution of this research is to analytically find the distortion of image in proximity printing due to diffraction effect and to verify the theory by experiments.