As device integration density increases exponentially predicted by Moore's law, power consumption becomes a bottleneck for system scaling. On the other hand, leakage power of on-chip cache occupies a large fraction of the total power budget. STT-RAM is a promising candidate to replace SRAM as on-chip cache due to its ultra-low leakage power, high integration density and non-volatility. However, building L1 cache with STT-RAM still faces severe challenges especially because of its high write latency and energy overheads. Moreover, intensive accesses in L1 cache accelerate oxide breakdown and threaten the lifetime of STT-RAM significantly. In this paper, we propose a Dynamic Overwriting Voltage Adjustment (DOVA) technique for STT-RAM L1 cache. A high write voltage is used for performance critical cache lines while a low write voltage is used for other cache lines to approach an optimal trade-off between reliability and performance. Experimental results show that the proposed technique can improve cache performance up to 18%, and 9% on average with almost the same reliability level as in the case when only the low write voltage is used.