A spectroscopic method was developed to diagnose electron temperature by neutral line spectrum ratio in various plasmas such as inductively coupled plasma (ICP), electron cyclotron resonance heated (ECH) plasma, and ohmic tokamak plasma. Detailed modeling work was performed for helium and hydrogen plasmas including complicated secondary processes such as imprisonment effect, excitation from metastable states, and excitation transfer. The modeling result shows that the two particular line spectrum intensity ratio is a function of electrondensity and temperature from which temperature information can be obtained as electron density is known. Line ratios were measured with four different types of hydrogen or helium plasmas. For a hydrogen plasma, $H_α$ and $H_β$ lines were used, and 388.9 nm and 501.6 nm lines were chosen for helium case. For comparison with other diagnostics, Langmuir probe was used at the same time. and the electron temperature determined by the line ratio measurement indicates a resonable agreement with probe data. This suggests that the line ratio method can be utilized to obtain temporal evolution of electron temperature in not only low temperature plasmas such as ICP but also higher temperature tokamak plasmas.