The laser machining is often accompanied by the generation of plasma phenomena. So study of plasma diagnostics will help to understand the physical mechanism of laser machining. In this paper, electron temperature of plasma was investigated theoretically and experimentally by laser-induced breakdown spectroscopy. The methods of line pair ratio, Boltzmann plot and Saha-Boltzmann plot were introduced to determine the electron temperature of T2 copper alloy. The experiment results showed that the precision of electron temperature calculated by line pair ratio method was poor while the Boltzmann plot and Saha-Boltzmann plot method which using several different lines lead to greater precision of the plasma temperature determination.