Abstract: Subcooled flow boiling in vertical rectangular channel under different pressure and heat flux was numerically investigated based on CFD.The phase change process was accomplished through the User Defined Function（UDF） to describe the mass and energy transportation,and the interface of liquid and vapor was captured by the Volume of Fluid（VOF） method.The results reveal that the interaction of evaporation and condensation in the cross section perpendicular to the flow direction forms the secondary flow,and enhances the natural convection near the wall.The bubble grows up in the sliding process,and merges with the adjacent ones to form bigger one,which has greater deformation.The bubble sliding enhances the heat transfer in the down stream and restrains the nucleation.With the higher pressure and lower heat flux,the bubble size,growth rate and the average void fraction at the outlet will be reduced.The simulation results of bubble growth and the wall temperature near the Onset of Boiling（ONB） agree well with the correlations in the literature.