Knowledge of the heat transfer phenomenon during a flow decay transient condition is important for the safety assessment of the very high temperature reactor (VHTR) during a loss of coolant accident. This study was conducted in order to investigate the transient convection heat transfer from a horizontal cylinder to helium gas under exponentially decreasing flow rate conditions. A platinum cylinder with a diameter of 1 mm was used as the test heater. A uniform heat generation rate was added to the cylinder by a power source. The cylinder temperature was maintained at an initial value under a definite initial flow rate of helium gas. Then, the flow rate of the helium gas started to decrease exponentially with different time constants m=m₀exp(t/τ), where m denotes the flow rate; m₀, the initial flow rate; t, time; and τ, the flow decay time constant. The surface temperature, heat flux, and heat transfer coefficients were measured during the flow decay transient process under wide experimental conditions such as initial flow rate and flow decay time constant. The initial flow velocity ranged from 10 m/s to 4 m/s, and the time constants ranged from 4.3 s to 15.4 s. It was found that the temperature of the test heater increased rapidly during this process, and that the increasing rate of the temperature was higher for a smaller time constant. The heat transfer coefficient versus time, during the flow rate decreasing process, was also obtained. The heat transfer coefficient decreased to a constant value at each flow decay time constant, and the rate of decrease was higher for a smaller time constant. Moreover, a three-dimensional numerical simulation was also conducted for this transient heat transfer process. The numerical results for the surface temperature increases in the test heater were compared to the authors' experimental data, and the transient heat transfer process during the flow decay transient process was clarified.