Shape memory characteristics and superelasticity of an equiatomic Ti-Ni alloy with surface sulfide layers were investigated by means of thermal cycling tests under constant load, tensile tests, and scanning electron microscopy. Martensitic transformation start temperature (Ms) increased by sulfidation, which is ascribed to a compressive stress imposed by surface sulfide layers. Surface sulfide layers were found to make transformation elongation be small when their thickness was less than 5 mu m. This is ascribed to the fact that the surface sulfide layers were not detached from substrates and constrained martensitic transformation. When thickness of the surface sulfide layers was 35 mu m, transformation elongation was not affected by them because they were detached during transformation. Superelastic recovery decreased by the sulfide layers when their thickness was about 5 mu m, while it did not change when the thickness was about 35 mu m.