This paper presents research that has sought to improve the in- cylinder air flow characteristics by using the guide vane swirl device (GVSD). Four guide vanes with various numbers of vanes were adopted to evaluate the performance and emission analysis before and after the installation to the normal engine. The investigation is performed by an experiment on a four-stroke single-cylinder D.I diesel engine test bed with four different types of guide vane swirl devices, were installed at the downstream of the air intake manifold. There by increasing the mixture quality of air & fuel in the combustion chamber before the initialization of ignition. The engine load tests were carried out at different loads with and without air swirl devices. This research found that five vanes was the optimized number of vanes since it decreased 21.2% of BSFC and 30.7% of NOX respectively and Brake thermal efficiency was increased 23.04% as compared with normal engine at full load.