A slab laser can reduce the effect of thermal-optical distortions due to its intrinsic phase-conjugate properties. It is hence a good laser structure. A rod laser with its side surface polished has Similar properties, because it has a special kind of resonator with side reflectors. Modes of such resonator are analyzed in this paper. The analytic results show that diffraction losses become smaller, and mode distributions become stronger in the central part of the end reflectors.