It is well known that a high-pressure fuel injection is effective for the reduction in particulates and smoke emissions. Exhaust gas recirculation (EGR) is effective for the reduction in NOX emission. In this study an experiment aiming to understand more comprehensive combustion under the condition of EGR and high-pressure fuel injection was carried out by using gas sampling method for the purpose of understanding what occurred inside the spray before and after combustion. The number of combustion cycles in this engine can be controlled in order to change EGR conditions by adjusting the residual gas concentration in the cylinder. Main results were: (1) Close to the nozzle tip, the sampling gas data showed little reaction which implies that combustion never occurs in this area during the injection period. (2) In the case of high-pressure fuel injection O2 concentration decreased faster and air dilution was more active and earlier. This may cause the decrease of smoke emission due to accelerated soot oxidation. (3) In the case of EGR, combustion was poor since oxygen concentration was insufficient. Thus, inactivity of oxidation reaction caused reduction in NOX. emission. (4) In the case of increasing the amounts of N2 gas while keeping the O2 content constant (same amount as without EGR), NOX. emission decreased without deterioration of smoke emission and Pmi.