In this paper, we consider energy minimization for multiprocessor system-on-a-chip (MPSoC) under lifetime reliability constraint of the system, which has become a serious concern for the industry with technology scaling. As today's complex embedded systems typically have multiple execution modes, we first identify a set of "good" task allocation and schedules for each execution mode in terms of lifetime reliability and/or energy consumption, and then we introduce novel techniques to obtain an optimal combination of these singlemode solutions, which is able to minimize the energy consumption of the entire multi-mode system while satisfying given lifetime reliability constraint. Experimental results on several hypothetical MPSoC platforms with various task graphs demonstrate the effectiveness of the proposed approach.