As the wireless network supports the mobility of a mobile station (MS), increasing MS power efficiency has become an important issue. In this thesis, we propose and analyze three power management schemes, the standard scheme, the threshold scheme and the semi-threshold scheme based on the sleep-mode operation for an efficient power management in the wireless packet networks and observe the effects of the operating parameters related to the power management schemes. As a performance measure for the power consumption, we propose the power consumption for switching per unit time per effective arrival and the total power consumption per unit time per effective arrival which consider the power consumption and the packet overflow probability simultaneously. In addition, since we consider delay-guaranteed services, the average packet transfer delay is also considered as a performance measure. Based on the performance measures, we obtain the optimal sleep-mode operation which reduces the power consumption per unit time per effective arrival with a given delay requirement. Numerical studies are also provided to investigate the system performance and to show how to achieve our objectives.