Abstract: The single-phase natural circulation of the main heat transport system (HTS) for CANDU 6 under shutdown condition was analyzed by using RELAP5 code, and the single-phase natural circulation mass flow rate model was developed. The comparison analysis of the Vijayan model and natural circulation models including the Chrichill-Chu model and McAdams model for RELAP5 was conducted, respectively. The results show that a lower horizontal heat transfer coefficient is obtained by using Vijayan model than that by the McAdams model, causing a slight higher cladding temperature. However, the vertical heat transfer coefficients from both models have no significant difference.