This paper presents a comparative analysis on the off-design performance of a gas turbine and organic Rankine cycle (GT-ORC) combined cycle under different operation approaches. A new solving procedure for off-design performance evaluation of the GT-ORC combined cycle was proposed and optimal design parameters for this GT-ORC combined cycle were given. Then simulations were conducted based on the mathematical model and assumptions. The inlet guide vane (IGV) control was applied to regulate the topping GT cycle. Results showed that the sliding pressure operation had an optimal sliding condition to make the GT-ORC combined cycle have the best off-design performance. This condition was a fixed superheat degree of 0.1 K for organic vapor, and the operation approach under such condition was named modified sliding pressure operation (MSPO). Simulation results showed the MSPO performed better than the constant pressure operation and bivariate operation under off-design conditions. Therefore, the MSPO might be more suitable for the off-design operation of the GT-ORC combined cycle. Finally, the sensitivity analysis indicated that the MSPO was more favorable for the GT-ORC combined cycle to reduce the influence of ambient temperature. (C) 2016 Elsevier Ltd. All rights reserved.