In the present work the thermal performance analysis of a simple open cycle gas turbine power plant which is situated at Uran in district Raigad of Maharashtra, India is carried out at varying ambient temperature and pressure ratio and a comparison is made from the stipulated ISO conditions. The analysis of the cycle has been carried out using software developed in C++. The results show that for every 50C rise in ambient temperature the gas turbine loses 0.44 % in terms of thermal efficiency and 0.5 MW of its useful power. Also there is a loss of 3.36 % in thermal efficiency at pressure ratio 9 as ambient temperature increases from 283 K to 313 K, this loss increases up to 3.89 % at pressure ratio 21, which reveals that, as pressure ratio increases percentage loss in thermal efficiency increases on increasing the ambient temperature. The thermal performance analysis reveals that, the ambient temperature and compression ratios are strongly influence the performance of gas turbine power plant.