Premixed charge compression ignition (PCCI) combustion has been studied for its homogeneous mixture formation and low combustion temperature with the aim of reducing both NOx and PM levels. This type of combustion also increases thermal efficiency due to an optimal air-fuel mixture. It is well known, however, that PCCI combustion reduces fuel economy because combustion tends to start before top dead center (TDC). Therefore, it is necessary to determine the optimal operating conditions for PCCI combustion in order to reduce NOx and PM and increase thermal efficiency. In this study, PCCI combustion was realized by adding an early-injection strategy to a conventional diesel engine system. The characteristics of PCCI combustion were analyzed via comparison to the injection strategy used in conventional diesel combustion. The results show that NOx and PM per unit of output power in PCCI combustion were reduced compared to conventional diesel combustion.