A series of experiments using a single-cylinder direct injection diesel engine was conducted to investigate the smoke reduction effect of post injection while varying numerous parameters: the post-injection quantity, post-injection timing, injection pressure, main-injection timing, intake pressure, number of injection nozzle orifices, and combustion chamber shape. The experiments were performed under a fixed NOx emission condition by selecting the total injection quantities needed to obtain the predetermined smoke emission levels without post injection. The smoke reduction effects were compared when changing the post injection timing for different settings of the above parameters, and explanations were found for the measured smoke emission trends. The results indicate that close post injection provides lower smoke emission for a combination of a reentrant combustion chamber and seven-hole nozzle. The same trend was also found in the tests that varied the injection pressure, main-injection timing, and intake pressure. However, a lower sensitivity of the smoke emission to the post injection timing was observed when using an injection nozzle with a larger number of orifices and a toroidal combustion chamber. The smoke reduction rate at the best post injection timing was higher for a lower injection pressure, larger number of nozzle orifices, and toroidal combustion chamber. The reasons for these trends were investigated, giving attention to the relation between the main spray flames and post sprays.