Line ratios and absolute intensities of helium singlet and triplet states are used to determine the electron temperature and number density of an electron cyclotron resonance heated (ECRH) plasma. Excitation rates and line ratios are calculated using the best available cross‐section information. An averaging technique based on absolute intersities is used to determine the temperature. This technique permits an experimental calibration of line ratios. In particular, the ratio of the 4713‐Å intensity to the 4921‐Å intensity is found to be strongly dependent on electron and neutral atom number densities. The ratio of the intensities of the 5048‐and 4713‐Å lines is also studied. These results show that the helium line ratios can be used to obtain accurate information on the temperature and number density of an ECRH plasma.