Transient forced convection heat transfer due to exponentially increasing heat input to a heater is important as a database for the safety assessment of the transient heat transfer process in a very high temperature gas-cooled reactor (VHTR). Forced convection transient heat transfer for helium gas flowing over a cylinder in a narrow channel was experimentally studied at various periods of exponential increases in heat input. The test heater was mounted horizontally along the center part of a circular test channel with a diameter of 5 mm. The heat generation rates of the heater,, was increased with the exponential function, . By using a narrow channel, relatively high flow velocity was achieved, and experimental data at a high Reynolds number were obtained. According to the results, the surface temperature and the heat flux exponentially increased with time. It was clarified that the heat transfer coefficient approached the quasi-steady-state for a period of more than approximately 1 s, and it reached higher values for a period of less than approximately 1 s. The heat transfer coefficients present high dependence on the flow velocity of the helium gas and the heater diameter. Heat transfer correlations at the quasi-steady state and the transient state were obtained based on the experimental data.