We have developed an ultrasonic injection molding (UIM) system, which applies ultrasonic waves to injection molding, as a precision injection molding technology. Molding of optical lenses showed that the lens weight increased when the ultrasonic wave was applied immediately after the resin was filled into a cavity. Results showed that, by applying ultrasonic waves, oscillatory flow was generated inside the cavity and consequently the weight of the lens was increased. The surface finish of the molded lens was also significantly improved in UIM when compared with that in conventional molding. The part of shrinkage during the packing and holding stages was suppressed by the oscillatory flow provided by the ultrasonic vibration. Because of ultrasonic energy absorption, local heating was generated inside the resin, resulting in the formation of oscillatory flow during packing and holding stages. Local heating, occurring especially between molten and skin layers, reduces the skin layer's deformation resistance. Consequently, replication during the packing and holding stages is facilitated by the UIM. Moreover, evaluation of the residual optical strain of concave lens revealed that the strain was much lower in UIM than in conventional molding. The decreased strain was attributed to the local heat generation by the ultrasonic waves. POLYM. ENG. SCI., 49:768-773, 2009. (c) 2009 Society of Plastics Engineers