Abstract: A key problem related with the tight lattice core design is whether it could be cooled successfully in a loss of coolant accident（LOCA）. In this paper, the modified code RELAP5/TIGHT was used to calculate and analyze the characteristics of large break loss of coolant accident of reactor cores with two different tight lattice design. It was found that compared with standard reactor core designs, the transient progress of a LOCA accident lasted a longer time for a tight lattice core design; the peak cladding temperature in the reflooding phase was much higher than that in the blown-down phase; the ratio of center distance between fuel rods to outer diameter of fuel rod（p/d） has a great impact on the peak cladding temperature; the increasing in the power density results in the increasing need to the safety injection flowrate. For safety consideration, p/d ratio less than 1.10 is not recommended.