Abstract: Departure from nucleate boiling (DNB) design basis is one of the most important bases for reactor core thermal-hydraulic design. In order to evaluate whether the DNB design basis meets the requirement of thermal-hydraulic design, the departure from nucleate boiling ratio (DNBR) design limit should be determined. The part-statistical methodology was described in detail. The part-statistical methodology and a reactor subchannel code were applied to calculate the DNBR design limit for 300 MWe nuclear power plant. The part-statistical methodology was used to replace STDP for DNBR analyses, and the results indicate that for complete loss of reactor coolant flow accident, about 5% DNBR margin can be got. The conclusions are presented to provide the key acceptable criterion for DNBR design basis.