Dynamic behavior of an actuator utilizing shape memory alloy is considered. The actuator is made of TiNi wire which is subjected to pretension and electrically heated. A one-dimensional thermoplastic model is assumed as a stress-strain curve of TiNi wire to discuss the effect of the prestrain on dynamic response. That is, it is assumed that the stress-strain curve is piecewise linear at each temperature around the phase transition points. The material constants of TiNi wire for modeling are determined by uniaxial loading tests at several temperatures. The relation of the prestrain and the maximum displacement is derived from the theoretical model, and it is compared with experimental results. The following results were obtained both theoretically and experimentally. (1) The displacement of the wire, which is the output of the actuator, depends on the prestrain and the heating temperature. (2) The displacement takes a maximum when the prestrain is in the stress-induced martensitic phase. (3) The maximum displacement increases as the heating temperature becomes higher, but there exists a definite limit.