This work presents the performance assessment of two industrial gas turbine power plants. The plants are GE M5001 gas turbine and BBC M05864 gas turbine, all operated in Rivers state, Nigeria. The international standard organization (ISO) rating, average operating field data and calculated results of the gas turbine performance parameters were compared. The first law analysis was employed for the calculations, where the exhaust gas temperatures, net power outputs and the thermal efficiencies were estimated. The calculations were done using the pressure ratios obtained from the field for the M5001 turbine, while the ISO value was used for the M05864 turbine. The maximum and minimum power generated as obtained from the field during the period of study are 20.37MW and 19.15MW respectively, obtained at pressure ratios of 8.33 and 7.87 respectively, while the corresponding calculated values are 24.44 MW and 24.17 MW respectively for the M5001 turbine. The corresponding thermal efficiency values calculated are 27.09% and 26.79% respectively. The ISO rating of the turbine is 26.84 MW, the thermal efficiency is 28.4% while the pressure ratio is 10.5. For the M05864 turbine, the ISO pressure ratio of 9.55 was used for the calculations. The calculated power output is 50.196 MW as against the field value of 50 MW. The ISO rating is much higher at 65 MW. There is thus no much difference in the net power output between the field data and the calculated value for the M05864 turbine, but there is significant difference in this parameter for the M5001 turbine. The ISO values are generally higher because the real engines operate at higher ambient temperatures. The comparisons between the calculated values and the field results reveal how well the engines are being operated.