The expression for intensity distribution and computer simulated images of screw dislocations with Burgers vector l/2 (111) viewed end-on in gernet crystals subjected to a long-range strain have been obtained by considering only the anisotropy of the photo-elastic property of the material. The computer-simulated images of end-on screw dislocations show two fold symmetry with a pair of dark and bright petals which result from the effect of a longe-range plane strain field superimposed on screw dislocation. The computer-simulated images are in good agreement with the experimental images in GGG crystals.