Comparison of the off design performance for the cogeneration systems utilizing single and two shaft gas turbine engines is presented in this study. Gas turbine performance is estimated using realistic compressor and turbine performance maps. There is no significant difference in part load thermal efficiency of single and two shaft gas turbines. Performance of the heat recovery steam generator is examined by reasonable modelling of the heat transfer process. Differences in the heat recovery performance of the two systems are analyzed with the aid of system parameters. The heat recovery efficiency of the two shaft system increases as power is reduced, while that of the single shaft system decreases. In the two shaft system, a larger temperature difference between gas and water overcomes the lower gas flow rate and leads to better heat recovery capacity than single shaft system. Total cogeneration efficiency of the two shaft engine does not decrease significantly as power is reduced and is higher than that of the single shaft engine at every part load.