The General Electric's LMS100, which combines heavy-duty frame and aeroderivative technology is a first modern production gas turbine system employing off-engine intercooling technology developed especially for the power generation industry. The external intercooler lowers air inlet temperature to the high-pressure compressor, causing its smaller power consumption and lower output temperature, which enables more effective cooling of the hot turbine parts. In the end it results in higher thermal efficiency, which is said to reach 46%. In the beginning of this diploma thesis the thermodynamic cycle of a gas turbine, its parameters and improvement possibility are presented. A description of the LMS100 and its features follows later. Subsequently, an analytical study is done to investigate the efficiency improvement by intercooling. The analytical formulae for dimensionless specific work and efficiency are derived and analysed. Next, the LMS100 is modeled by means of the commercial plant performance software GateCycle. The obtained results are presented and analysed.