Dislocations in an epitaxial GaN film with a (1010) surface (m-GaN) were investigated by means of X-ray diffractometry. It was clarified that a-type screw dislocations existed in the sample by evaluating diffraction contrast on X-ray topographs (XRTs). In addition, local lattice inclinations of the (1010) plane toward the [0001] direction were observed with spatial intervals similar to the separation distances of those a-type screw dislocations slipping on the (0001) basal plane and also on other cross-slip planes in the sample. (C) 2015 The Japan Society of Applied Physics