Abstract: Single-phase water characteristics of flow and heat transfer in steady and transient conditions were experimentally investigated in the horizontal and vertical rectangular channels with the pressure ranging from 0.5MPa to 5.0MPa..The experimental results indicated the characteristics of single-phase water flow and heat transfer in transient conditions were similar to that in steady conditions.So were the experimental results for horizontal and vertical narrow rectangular channels.The single-phase water frictional resistance coefficient in narrow rectangular channel were larger than that of normal channels,and the transition region from laminar flow to turbulence flow had a small Reynolds number relative to conventional channels.The critical Reynolds number at the transition region was in the range900 ≤ Re≤ 1300.Based on Dittus-Boelter correlation,the experimental correlations of heat transfer were obtained,and the coefficients of the new experimental correlations were less about 11.3% than that of Dittus-Boelter correlation.In steady conditions,the heat transfer coefficient at turbulence flow region increased observably with the mass flux,it also increased slightly with the heat flux,and it had slight relationship with the pressure.