This study is aimed to clarify transient heat transfer process between the surface of solid and the neighboring helium gas in Very High Temperature Reactor (VHTR) or Intermediate Heat Exchanger (IHX). In this paper a series of plate heaters with different widths under various pressures inside a circular channel have been tested for forced convective flow. The heat generation rate of the plate heater was increased with a function of Q(0)exp(t/tau) (where t is time and tau is period of heat generation rate or e-fold time). The heaters were platinum plates with a thickness of 0.1 mm and widths of 2 mm, 4 mm and 6 mm. The heat flux, surface temperature, and transient heat transfer coefficients were measured for helium gas passing by horizontal plates under wide experimental conditions such as velocities, pressures, widths of plate and periods of heat generation rate. It was clarified that the heat transfer coefficient approaches the quasi-steady-state value when the period is more than around 1 s and it becomes higher when the period is shorter than around 1 s. It was found that the heat transfer coefficient becomes higher with the increase of gas pressure under the same velocity and period of heat generation rate. It was also found that the widths have a great influence on the heat transfer coefficient: the heat transfer coefficients of narrow heaters are much higher than those of wide ones under the same velocity and the same pressure. Based on the experimental data, empirical correlations were obtained for both quasi-steady state heat transfer and transient state one at various plate-widths. (C) 2014 Elsevier Ltd. All rights reserved.