Diffusion of Cu and Ga atoms in Cu-poor CuGaSe2 (CGS) has been investigated by first-principles calculations with the generalized gradient approximation (GGA). The activation energies and migration pathways were obtained by means of the combination of linear and quadratic synchronous transit (LST/QST) methods and nudged elastic band (NEB) method. The activation energy of Cu migration in CGS was 0.94 eV, which was slightly lower than Cu migration in CuInSe2 (CIS). The activation energy of Ga migration in CGS was 1.89 eV, which was considerably larger than In migration in CIS. The transition states of Cu and Ga migrations are discussed on the basis of the interactions between the migrating atom and the neighboring atoms.