Abstract: In order to investigate the natural circulation characteristics and inherent safety features of the natural circulation lead-bismuth cooled fast reactor, the unprotected loss of heat sink (ULOHS) and protected loss of heat sink (PLOHS) of 10 MW natural circulation lead-bismuth cooled fast reactor were simulated by NTC-2D. As for the ULOHS, the temperatures of coolant, cladding and fuel pellet are far below the safety limits, and the reactor shuts itself down on account of the strong negative temperature feedback of reactivity. With regard to PLOHS, the reactor is safe during 600 s after the accident with the help of reactor trip protection system. The simulations of the two transient indicate that the lead-bismuth cooled fast reactor possesses good natural circulation characteristics and inherent safety features.