The leakage inductance identification of power transformer can be regarded as the basis of fault identification. The accuracy of leakage inductance identification is the key of transformer fault identification. Therefore, according to equivalent circuit equations, leakage identification methods of single-phase transformer and three-phase transformer based on least square method were proposed in this paper. The relative error of leakage inductance measurement using EMTP-ATP simulation is below 0.2%. Transformer experiments were conducted on a single-phase transformer, which is used in dynamic simulation laboratory. The relative error of the test results compared with the calculating results on the light of nameplate is 3.84%. In addition, leakage inductance were measured under different frequency and different saturation level. Results show that the leakage inductance of transformer can be accurately measured by leakage inductance identification method based on least square method, and the leakage inductance is not sensitive to frequency and saturation level.