A method of inspecting defect of a mask is provided. In this method, a database for storing a plurality of virtual simulation models is created. The virtual simulation models are determined by a plurality of factors including an optical effect and a chemical effect during the transferring the pattern of a mask to the photoresist layer on a wafer. A mask defect image is acquired. A simulation contour of the mask defect image is generated from at least one virtual simulation model in the database. Next, the acceptability of the mask is determined.