This study presents numerical investigation on the performance of S-shaped air intake normal and aggressive diffuser with 22 length reduction. Both models have same area ratio of 3.1 with different total length, turning angle and radius of curvature. The numerical investigation was implemented using CFD simulation by ANSYS-FLUENT 15 software. The inlet Reynold number was 4�104 and turbulence intensity 4.1. The performance evaluation was performed throw evaluation the static pressure coefficient, pressure loss coefficient, distortion coefficient and static pressure wall coefficient. The numerical results show that the performance in the case of aggressive S-shaped diffuser has been reduced compared to the normal S-shaped diffuser. This reduction resulting from the early flow separation and increase of the separation zone due to the high curvature of top and bottom surfaces of aggressive S-shaped diffuser. The results show that the static pressure recovery coefficient decreased by 31, the total pressure loss coefficient and distortion coefficient increased by 9.5 and 8.2, respectively, compared to the S-shaped diffuser. The static pressure wall coefficient on the top and bottom surfaces was dropped with the aggressive S-shaped diffuser. © 2018 The Authors, published by EDP Sciences.