Effects of precipitates of C11_b-type structure formed by aging on the mechanical properties of a shape memory Ti–40.5Ni–10Cu alloy, which slightly off-stoichiometric to the Ti poor side, were studied by constant-load thermal cycling tests, conventional tensile tests and transmission electron microscopy. The shape change under a constant load, associated with thermally induced B2-B19 transformation, occurs in narrower temperature range for both the cooling and heating processes in the fully aged specimen compared to the as-solution-treated specimen. The total elongation due to thermally induced B2-B19 transformation under a constant load decreases and the 0.5% offset stress in the conventional tensile test increases with increasing aging time. These aging effects on mechanical properties are caused by the precipitates of C11_b-type structure. The precipitates act as obstacle to the rearrangement of variants of the B19-martensite.