Abstract: The RELAP5 best-estimate code was used to perform small break loss of coolant accident (SBLOCA) for the ACME (advanced core-cooling mechanism experiment) facility built in China. Thereafter, uncertainty quantification evaluation was performed, including the selection of input uncertain parameters, the application of Wilks nonparametric statistics and the calculation of uncertainty propagation based on SNAP interface. Furthermore, the results of uncertainty and sensitivity analysis were discussed. The 95/95 uncertainty bands of key output parameters were obtained, and the lower band of minimum core level is maintained still above the top of the active core section, thus the core uncover does not occur with 95% confidence. Important uncertainty contributors for the minimum core level are identified through sensitivity analysis.