The purpose of the present work is to study the sensitivity of uncertainty parameters of critical flow on analysis of LOCA with MARS-KS (Multi-dimensional Analysis for Reactor Safety) used by KINS as independent regulatory assessment. In this paper, the uncertainty of critical flow model is considered by three parameters, thermal non-equilibrium factor, discharge coefficient and length to diameter ratio and it is determined uncertainty ranges by using large-scale Marviken experimental data. Break flow is strongly dependent on thermal non-equilibrium in the ruptured pipe with small L/D ratio, and appropriate thermal non-equilibrium values are provided in this study by analyzing results between Marviken experimental data and MARS-KS code. Additionally, a correlation for the relation between quality at the throat and L/D ratio was developed using the effect of quality with non-equilibrium on the critical mass flow rate. It also evaluated LBLOCA of APR1400 focused on the effect of three uncertainty parameters that affect PCT and the break flow. Results of plant analysis showed that deviation of PCT was 80.5ºC in the uncertainty range owing to super-heated liquid causing from the thermal non-equilibrium.