Health monitoring data, namely power output and exhaust gas temperature are used for performance analysis of an industrial gas turbine engine. Apart from ambient temperature, influences of fuel consumptions and compressor fouling on the engine performance are also considered in the work. A model-free data analytics approach is used to study the effects of individual governing factors. A performance index (ratio of power generation to fuel consumption) is proposed as the metrics for monitoring the engine performance. The step-by-step analysis suggests that the engine performance shows two opposing trends with the ambient temperature. Fouling is also clearly shown to reduce the engine performance using the proposed indices.