Abstract: The start-up physical test of 18-months refueling for unit 1 of Hongyanhe Nuclear Power Plant has completed. The results show that the predicted value of 18-months refueling theory is in good agreement with the measured results. The results verified the accuracy of refueling design. In this paper, the results from the startup physical tests after 18 months refueling and the 12 months refueling were compared, and the changes of core characteristics for 18-months refueling core are pointed out and analyzed.