Online compressor washing is a promising method of preventing/ recovering the effects of fouling on the compressor blades. However, proper strategies need to be implemented for online compressor washing to be effective since it is conducted when the engine is in operation. This study presents an experimental investigation of wash fluid preheating on the effectiveness of online compressor washing in a suction wind tunnel compressor cascade. Crude oil was uniformly applied on the compressor cascade blade surfaces using a roller brush, and carborundum particles were ingested into the tunnel to create accelerated fouled blades. Demineralized water was preheated to 500C using the heat coil provided in the tank. Washing of the fouled blades were conducted using single flat fan nozzle, where the preheated and non preheated demineralized water were used separately to wash the fouled blades. When fouled blades washed with preheated demineralized and the non-preheated were compared, it was observed that there was little or no difference in terms of total pressure loss coefficient and exit flow angle. However, when the fouled and washed cases were compared, there was a significant difference in total pressure loss coefficient and exit flow angle.