[en] The present study is to assess the applicability of the best-estimate thermal-hydraulic codes, MARS 1.4, for analysis of thermal-hydraulic behavior in PWRs during natural circulation conditions. The code simulates a BETHSY test 4.1a, which was conducted in the integral test facility of BETHSY. The test represents the cooling states of the primary cooling system under two-phase natural circulation and reflux condensation mode with conditions corresponding to the residual power, 2 % of the rated core power value, and a constant pressure of 6.8 MPa at the secondary system. Based on MARS calculations, the major thermal-hydraulic behavior during natural circulation is evaluated and the differences between the experimental and calculated results are identified