Onset of nucleate boiling and flow boiling heat transfer are experimentally investigated in a 5 x 5 rod bundle at low flow rate under the pressure of 6 MPa. The experimental mass flux ranges from 25 to 200 kg/m(2) s, heat flux varies from 25 to 75 kW/m(2), the maximum vapour quality is 0.45. Effects of parameters on ONB and flow boiling are examined. The wall superheating at ONB point increases with the rise of heat flux while decreases with the increase of mass flux. The effects of mass flux and quality on flow boiling heat transfer coefficients are not distinct. For both subcooled and saturated flow boiling in the 5 x 5 rod bundle, the effect of heat flux on heat transfer is more remarkable at high flow rate than at low flow rate. Comparisons between predictions by existing empirical correlations and the experimental data are conducted. Basu's correlation could predict the wall superheating at ONB point within a deviation less than 30% under conditions of higher mass flux (>150 kg/m(2) s). As for the comparisons in flow boiling regions, predictions by Chen's correlation agree well with the experimental data in both sub cooled and saturated regions, except under conditions of low mass flux with low heat flux. Most of the correlations for boiling heat transfer and ONB are derived from the experimental data obtained at high flow rate, however, current experiments indicate that these correlations cannot be directly used under low flow rate conditions. (C) 2019 Elsevier Ltd. All rights reserved.