Abstract: Experimental researches and numerical analysis on the heat transfer characteristics of forced circulation flow in a circular tube under different inclined conditions were introduced in this paper.For the single-phase flow,the experimental results show that the heat transfer around the tube was unsymmetrical under the buoyancy effect,and the heat transfer was weakened at the upside of tube to increase the wall tem-perature,while the heat transfer was enhanced at downside to decrease the wall temperature.The numerical calculation results show the same phenomena.Based on the experiment,a modified factor for the inclined heat transfer coefficient was presented in this paper,to estimate more accurately the heat transfer variation in single phase convection at inclined condition..Stresses on the bubble in two-phase flow was analyzed,to il-lustrate the why the effect of inclined condition on the heat transfer is not obvious.