Abstract: The prediction of turbine off-design performance by streamline curvature method is explained. "The performances of the 4000 HP. Locomotive gas turbine have been evaluated. The evaluated data are in good agreement with the experimental results of the gas turbine. The performances of the turbine with turnable guide vanes have also been evaluated. The effect of turnable guide vanes on locomotive gas tractive turbine and off-design operation of the gas turbine unit is studied.