Abstract: Departure from nucleate boiling (DNB) design basis is one of the most important basis for reactor core thermal-hydraulic design. In order to evaluate whether the DNB design basis meets the demand of thermal-hydraulic design, the departure from nucleate boiling ratio (DNBR) design limit should be determined first. The RTDP methodology was described detailedly, in which the uncertainties of operating parameters and nuclear design parameters were statistically combined. Then the RTDP methodology and a reactor subchannel code were applied to calculate the DNBR design limit and quality limit for LPP. The conclusions were presented to provide the key acceptable criterion for DNBR design basis.