Transient and quasi-steady state heat transfers were measured in vertical small tube with exponentially increasing heat inputs．The flow velocities，the inlet liquid temperatures and the inlet pressure were ranging from 1.0 to 7.5 m/s，295 to 355 K and 645 to 858 kPa．The forced convection water flow test loop made of stainless steel consists of a multistage canned-type circulation pump with high pump head，a heater，a cooler，a flow meter，a pressurizer，an expansion tank，an ion exchanger and a test tube．The heat generation rate was raised through an exponential function，Qoexp (t /τ)，where Qo is an initial heat generation rate, t represents time and τ is e-folding time．It was obtained that the heat transfer coefficient increased with the decreasing of inner diameter of test tube．The heat transfer coefficient also increased with decreasing of the e-folding time of the heat generation rate at a flow velocity．In spite of the e-folding time，the heat transfer coefficient increased with an increase in flow velocity．Moreover, the Nusselt number under the transient condition was affected by the Fourier number and the Reynolds number．
