An inspection apparatus and a method for detecting defects in a substrate having a semiconductor device thereon are provided. The method includes establishing a first inspection region including first patterns repeatedly formed in a first direction and a second inspection region including second patterns repeatedly formed in a second direction on the substrate, determining a first unit inspection size of the first inspection region and a second unit inspection size of the second inspection region, obtaining images of the first and second patterns by moving the substrate in the first direction, and detecting defects in the first and second inspection regions by comparing the obtained images of portions of the first and second inspection regions, respectively, with each other. The first inspection size and second inspection size function as comparison units if defects are detected. The substrate may face an image receiving member.