In situ high resolution neutron diffraction techniques were used to investigate martensitic transformation between the cubic austenite phase and hexagonal R-phase in the NiTi shape memory alloy. Thermally induced transformation and reorientation of the R-phase variants upon loading in tension and compression were studied. Rhombohedral distortion of the R-phase, the asymmetry in R-phase variant selection under stress, macroscopic stress-strain behavior and values of diffraction elastic constants in tension and compression are discussed.