An equation of state, which can best represent the P-V-T data ethylene gas below the critical point （9.5 ℃, 51 bar）, and its coefficients were determined by means of the non-linear least-squares technique using recently published experimental data. A total of 90 data points, ranging 243-297 ^oK temperature and below 50 bar pressure, were fitted to the modified Bender equation and the coefficients of the 6-parameter and 10-parameter equations were obtained. The RMS deviations of the calculated densities are 0.1 % from the experimental data used in this curve fitting, and 0.15 % from Michels data. Therefore these equations of state can be used within the experimental range with a probable error of ±0.15 %.