Abstract: Fast reactor is important for the sustainable development of nuclear energy due to its good breeding ability. In this paper, two conceptual designs of fast reactor using metal fuels are proposed, respectively, based on the once-through and closed fuel cycle. For the case of the closed fuel cycle, the reactor is designed to reach the breeding ratio of 1.4 with the double time of 11 years. Meanwhile, the case of once-through fuel cycle is designed to operate with long cycle. The in-core breeding and burn is realized by using the fuel shuffling for 38 years. Based on the two cases, the differences of designing fast reactor for different fuel cycle are compared in the view of core physical parameters, resource use ratio and economics.