Abstract: For the parameter identification of transmission line, there often exist the gross errors in the measured data, and it is lacking of robustness with the traditional least square estimation. In this paper, the IGG (Institute of Geodesy & Geophysics, Chinese Academy of Sciences) robust estimation is applied to identify the transmission line parameter, and parameter identification of transmission line by robust least square estimation is proposed based on IGG robust criterion. In detail, robust criterion is introduced by IGG method through renovation of objective function based on circuit linear mathematical model of dual terminal PMU (Phasor Measurement Units) data and corresponding least square identification. Therefore, the right of effective observation information can be protected and cut down, and harmful observation information is denied, which make modified least square estimation has stronger robustness. The simulation results based on PSCAD data verify the effectiveness, noise immunity ability and robust ability of the proposed method. Furthermore, the results based on the field PMU data show the practicability of the proposed method.