The widespread use of mobile devices in the IP network has lead to a new attempt to apply a power-saving mode (PSM) to real-time traffic such as Voice over IP (VoIP). This paper evaluates the performance of the PSM when the PSM is used for VoIP services of mobile devices. Taking the activity of each conversational party into account, we consider two different kinds of PSMs: one is employed during the talk-spurt periods and the other is employed during the mutual silence periods, of two conversational pal-ties. The performance of each PSM is analyzed with respect to buffering delay, the probability of packet drop, and power consumption of a mobile VoIP device. Thereafter, the maximum bound of sleep interval in each period is derived, which minimizes the power consumption of the mobile device without violating the quality-of-service (QoS) of VoIP. In the various network environments, the analysis and simulation results show that the proposed PSM for VoIP Significantly decreases the power consumption while satisfying the end-to-end delay and packet drop probability constraints of a VoIP connection(1).