The lifetime of wireless sensor networks (WSNs) is an important issue of much recent researches. A practical and widely used approach to prolong the lifetime of WSN is multi-mode protocols in which the sensor nodes are switched between active and sleep modes. But there is no work so far to study the multi-mode structures from reliability viewpoint. In this paper two terminal reliability of WSN is calculated, using Monte Carlo Simulations (MCS). Then via simulation results the effects of sleep probability on consumed energy and network reliability are considered. As we expect increasing the sleep probability prolong the lifetime of WSN but lead to long path length from source to sink and so decrease the message reliability.