This paper presents a novel architectural mechanism and a power management structure for the design of an energy-efficient Gigabit Ethernet controller. Key characteristics of such a controller are low-latency and high-bandwidth required to meet the pressing demands of extremely high frame and control data, which in turn cause difficulties in managing power dissipation. We propose a flow-through-queue (FTQ) based power management method, which allows some of the tasks involved in processing the frame data to be offloaded. This in turn enables utilization of multiple clock rates and multiple voltages for different cores inside the Ethernet controller. A modeling approach based on semi-Markov decision process (SMDP) and queuing models is employed, which allow one to apply mathematical programming formulations for energy optimization under performance constraints. The proposed Gigabit Ethernet controller is designed with a 130nm CMOS technology that includes both high and low threshold voltages. Experimental results show that the proposed power optimization method can achieve system-wide energy savings under tighter performance constraints.