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 the test heater and heated by electric current. The heat generation rate was exponentially increased with a function of Q(o) exp(t/tau). The gas flow velocities ranged from 4 to 10 m/s, the gas temperatures ranged from 290 to 353 K, and the periods of heat generation rate, tau, 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 tau longer than about 1 s, and it becomes higher for the period shorter than around I s. Empirical correlation for transient heat transfer was also obtained based on the experimental data.