In this paper, we introduce the use of slow-silent virtual channels to reduce the switching power of on-chip networks while keeping the leakage power small. Adding virtual channels to a network improves the throughput until each link bandwidth is saturated. This enables us to reduce the switching power of on-chip networks by decreasing their operating frequency and supply voltage. However, adding virtual channels increases the leakage power of routers as well as the area due to their large buffers; so the runtime power gating is applied to individual virtual channels to eliminate this problem. We evaluate the performance of slow-silent virtual channels by using real application traces, and their power consumption (switching and leakage) is evaluated based on the detailed design of a virtual-channel router placed and routed with a 90nm technology. These evaluation results show that a network with three or four virtual channels achieves the best energy efficiency in a uniform traffic. In the cases of neighboring communications, a network with two virtual channels is better than the other networks with more virtual channels, because the performance improvement from no virtual channel to two virtual channels is the largest and their frequency and supply voltage can also be reduced well in these cases.