The diesel particulate filter (DPF) is an effective technology for particulate matter (PM) and particle number (PN) reduction. On heavy-duty diesel engines, the passive regeneration by Diesel Oxidation catalysts (DOC) and catalyzed DPFs (CDPF) is widely used for its simplicity and low cost, which is generally combined with the active regeneration of exhaust fuel injection.This study investigated a DOC-CDPF system with exhaust fuel injection upstream of the DOC. The system was integrated with a 7-liter diesel engine whose engine-out PM emission was below the Euro IV level and tested on an engine dynamometer. PM and PN concentrations were measured based on the Particle Measurement Programme (PMP), and the number/size spectrum for particles was obtained by a Differential Mobility Spectrometer (DMS). The filtration efficiency of DPF on PN was higher than 99% in ESC test, while the efficiency on PM was only 58%. During the active regeneration, a certain amount of diesel fuel was injected into the tailpipe and then oxidized in the DOC. The exotherm of the fuel oxidation, the temperature distribution of the DOC, and the concentrations of gaseous emissions were investigated by analyzing the effects of various factors including fuel injection rate, injector location, DOC inlet temperature and exhaust mass flow rate. Increasing fuel injection rate led to hydrocarbons (HC) slip downstream of the DOC at high space velocity (SV). N2O was generated as a by-product during diesel fuel oxidation in the DOC, especially at the beginning of fuel injection.