Abstract: An experimental investigation on onset of nucleate boiling (ONB) with the deionized water as experimental medium in the rod bundle channel was performed in this paper. The experiments were carried out at inlet temperature ranging from 80 to 100 ℃, the mass flow rate from 0 to 100 kg/(m²•s), and heat flux varying from 0 to 80 kW/m². Effect on ONB of some parameters, including part of thermal parameters and geometry parameters of rod bundle, was also analyzed. Meanwhile, by introducing the Reynolds number, the empirical correlation of ONB suitable for the rod bundle channel was obtained by the nonlinear regression analysis for ONB’s experimental data. The results show that the new empirical correlation is good at predicting ONB’s heat flux for rod bundle channel and the relative error of predicting value is 14.75%.