The recent development of molten salt fast reactors has generated a renewed interest in them. As compared to traditional solid fuel fast neutron systems, it has many unique advantages, e.g., lower fissile inventory, no initial criticality reserve, waste reduction, and a sim-plified fuel cycle. It has been recognized as an ideal reactor for achieving a closed Th–U cycle. Based on the carrier salt, molten salt fast reactors could be divided into either a molten chloride salt fast reactor (MCFR) or a molten flu-oride salt fast reactor (MFFR); to compare their Th–U cycle performance, the neutronic parameters in a breeding and buing (B&B) transition scenario were studied based on similar core geometry and power. The results demon-strated that the required reprocessing rate for an MCFR to achieve self-breeding was lower than that of an MFFR. Moreover, the breeding capability of an MCFR was better than that of an MFFR;at a reprocessing rate of 40 L/day, using LEU and Pu as start-upfissile materials, the doubling time (DT) of an MFFR and MCFR were 88.0 years and48.0 years, and 16.5 years and 16.2 years, respectively. Besides, an MCFR has lower radio-toxicity due to lower buildup of fission products (FPs) and transuranium (TRU), while an MFFR has a larger, delayed neutron fraction with smaller changes during the entire operation.