The microstructure and the M_s temperature were systematically investigated for three types of Ti–Ni–Nb shape memory alloys, Ti_50−x⁄2Ni_50−x⁄2Nb_x, Ti₅₀Ni_50−xNb_x and Ti_50−xNi₅₀Nb_x covering a wide composition range, by utilizing optical microscopy, SEM, EPMA, X-ray diffraction and DSC. Consequently, the following results were obtained. Addition of Nb promotes phase separation. In Ti_50−x⁄2Ni_50−x⁄2Nb_x and Ti₅₀Ni_50−xNb_x alloys, the microstructure changes with increasing Nb addition in the following way: B2 phase → primary B2 phase+eutectic → eutectic → primary bcc phase+eutectic. A pseudo-binary “TiNi–Nb” phase diagram was proposed to explain the above behavior. In Ti_50−xNi₅₀Nb_x alloys, the microstructure was the primary B2 phase only within the observed composition range, but slight micro-segregation of Nb was observed. The M_s temperatures of Ti–Ni–Nb ternary alloys were measured for the three types of alloys as a function of Nb addition, and the result was rationalized, based on the observations that the M_s temperatures in the alloys are mainly dependent on the Ni/Ti ratio in the B2 phase.