Diesel Particulate active Reduction system (DPR) is a system that traps particulate matter in diesel exhaust gas with a particulate filter and actively regenerates the filter when PM accumulates to a specific level. In 2003, DPR was installed on Hino's light-, medium-, and heavy-duty diesel engines, and about 50,000 units of these DPR-equipped diesel engines are currently on the market. This paper reports results of further progress made on optimization of the active regeneration function of DPR.The goal of successful development of DPR is to optimally control the system under various engine-operating conditions to regenerate the filter without producing abnormal combustion of PM and to minimize the amount of unburned PM to keep the filter from clogging.To improve the control of DPR, the combustion phenomena of PM collecting on the filter were studied through visualization, and the factors influencing combustion were determined. Based on quantitative information obtained in the captured images of combustion, an empirical formula was developed to calculate the combustion rate of particulate accumulating on the filter. The formula was verified through engine tests and used to optimize the control of DPR.