[en] A multi-dimensional transient analysis during the LBLOCA of the Kori Unit 1 has been performed by using the MARS code. Kori Unit 1 has upper plenum injection type ECCS for the low pressure safety system. Up to now, the analysis of Kori Unit 1 LBLOCA has been conducted by using KREM based upon the WCOBRA/TRAC-SECY-UPI method. This method has been approved for the Westinghouse 3 loop reactor type which uses the cold leg injection. This paper suggests the analysis results obtained by the multidimensional reactor vessel modeling. The results are focused on the hydraulic behavior at the upper plenum and core region multi-dimensional analysis method by using the MARS