This paper studies the effect of Ta addition on the martensitic transformation characteristics and the X-ray visibility of NiTi shape memory alloys. It was found that: (1) The transformation temperatures of the Ti 49 Ni 51 binary alloy increase drastically by an addition of 0 -- 4at%Ta, but only slightly when the concentration exceeds 4at%. (2) The addition of Ta greatly decreases the sensitivity of the martensitic transformations to the variation in the Ni-Ti ratio. (3) The addition of Ta to a TiNi binary alloy can improve its X-ray visibility.