In this paper, an algorithm named MuVoF is proposed to perform multivoltage multifrequency low-energy high-level synthesis for functionally pipelined datapath under resource and throughput constraints. A datapath is partitioned into a number of pipelined stages such that the clock period can be extended maximally. A multivoltage assignment algorithm then utilizes the extended clock period to reduce energy by lowering the supply voltages of the resources. The results are further refined by four local transformations performed in an iterative process. The experiment results show that MuVoF is capable of exploring the design space effectively and achieves efficient energy reduction.