Abstract: Based on the VOF model in Fluent, a sub-cooled boiling model was established and was inserted by UDF code into the control equations. Sub-cooled boiling in the 5×5 rod bundles channel with spacer grid was simulated. Heat transfer characteristics in sub-channels were analyzed by investigating the void fraction distribution and the heat transfer characteristics of the fuel rods. The results show that the void fraction distribution is different even if the heating condition is the same, and the void fraction is larger in the edge channel than that in the angular channel. The analysis of the heat transfer characteristics around the rod shows that, the heat transfer of the direction of 0°, 90°, 180°, 270° of fuel rod is better, the x velocity is larger, and the boiling is stronger.