The Hugoniot data and shock temperatures of gaseous helium were measured in the pressure range 500–700MPa and temperature 6.7–10.8kK generated by flyer accelerated up to ∼6km/s with a two‐stage light‐gas gun. Gaseous specimens were shocked from initial pressure 5MPa at 285K. Spectral radiance histories from the shocked helium sample were studied experimental by using pyrometer. Shock velocity was measured and particle velocity was determined by the shock impedance matching method. The equation of state and degree of ionization has been calculated by applying the Saha model with Debye‐Hu¨chel correction. In comparison of the experiment with theoretical results, we found that shock pressure versus particle velocity is in agreement with each other, but for the shock temperatures. It implies that the theoretical degree of ionization should be lower than that of the experiments due to their lower values of shock temperature, and thus the ionization energies and level populations of the compressed atoms are also changed. © 2004 American Institute of Physics