This letter proposes a novel medium reservation MAC protocol that reduces both end-to-end delay and energy consumption for wireless sensor networks. MRMAC reduces endto-end delay by informing the intended receiver of (next packet arrival time) and (medium reservation information) to reserve the medium in advance. Through this medium reservation phase, following packets can be delivered with reduced delay in multihop streaming phase. A simulation shows that the result of the proposed protocol outperforms previous works.