Transient heat transfer coefficients for helium gas flowing over a horizontal plate (ribbon) were measured under wide experimental conditions. The platinum plate with a thickness of 0.1 mm was used as test heater and heated by electric current. The heat generation rate was exponentially increased with a function of Q₀exp(t/τ). The gas flow velocities ranged from 4 to 10 m/s, the gas temperatures ranged from 17 to 80 %C, and the periods of heat generation rate, τ, ranged from 50 ms to 17 s. The surface superheat and heat flux increase exponentially as the heat generation rate increases with the exponential function. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period τ longer than about 1 s, and it becomes higher for the period shorter than around 1 s. The dependence of transient heat transfer on the gas flowing velocity becomes weaker when the period becomes very shorter. The gas temperature in this study shows little influence on the heat transfer coefficient. Semi-empirical correlation for quasi-steady-state heat transfer was obtained based on the experimental data. The Nusselt numbers for the plate are lower than those of authors' previous experimental data using a horizontal cylinder. The ratios of transient Nusselt number, Nu_tr, to quasi-steady-state Nusselt number, Nu_st, at various periods, flow velocities, and gas temperatures were obtained. The heat transfer shifts to the quasi-steady-state heat transfer for longer periods and shifts to the transient heat transfer for shorter periods at the same flow velocity. It also approaches the quasi-steady-state one for higher flow velocity at the same period. Empirical correlation for transient heat transfer was also obtained based on the experimental data.