Abstract: Abstract: Diesel engines are used extensively due to their superior economy and strong power performance, but the air pollution these engines produced is hard to overlook. With regulatory legislations for diesel engine emissions becoming stricter worldwide, integrated after-treatment systems are becoming increasingly important. The reduction performance of after-treatment systems can be affected by many factors. In this study, we investigated the effects of diesel oxidation catalytic converter (DOC) catalyst composition on gaseous emissions characteristics for the after-treatment systems. The after-treatment systems consisted of a diesel oxidation catalytic converter (DOC), a catalyzed diesel particulate filter (CDPF), and a selective catalytic reduction (SCR). An engine dynamometer test was conducted on a light-duty diesel engine which equipped with DOC + CDPF + SCR systems. All DOCs used were coated with Pt-Pd catalyst material. The experiment was performed under steady-state conditions of Yunnei D20TCI diesel engine with a displacement of 1.9 liters. This experiment used four sampling points located between the front and rear ends of each after-treatment device. During the full-load test, the data were collected every 200 revolutions from 1 000 to 3 200 r/min. The partial load test was conducted at the maximum torque speed (2000 r/min) with five loads: 10%, 25%, 50%, 75% and 100%. During the test, the data of different Precious Group Metal (PGM) load and Pt/Pd ratio of DOC were collected. Five sets of DOC catalyst composition were used: 2120 (Pt/Pd, 5:1), 1410 (5:1), 880 (5:1), 880 (7:1), 880 (10:1) g/m3. The results showed that the light-off temperatures (T50) of carbon monoxide (CO) and total hydrocarbon compounds (THC) elevate with the decrease of PGM load. And the T50 of CO and THC both increased with the growth of Pt/Pd ratio. No significant gap existed between the various DOC catalyst composition, indicating that the Pt/Pd ratio of DOC had little influence on the engine's economic performance. The CO oxidation activities increased monotonically as the PGM load, or Pt/Pd ratio increased in a low-temperature range (< 300 ℃), but this beneficial effect mitigated as the temperature was enhanced continuously (≥ 300 ℃). The trends of oxidation activity of THC were similar to CO. Nitric oxide (NO) oxidation was enhanced with higher PGM load. NO oxidation activity was more sensitive to Pt element which can amplify it, exhibiting an opposite trend to CO and THC. But the maximum NO conversion did not reach 50% for all DOCs. High temperature was adverse to NO reaction since it was exothermic. When the SCR inlet temperature was low, high nitrogen dioxide (NO2) concentration can increase the SCR nitrogen oxides (NOx) reduction. NOx reduction was above 90% without large gap when the SCR inlet temperature is high. Along with the flow direction, the average temperature drop in the SCR was the largest, followed by the DOC. The average NO2/NOx ratio increased by 23.9% through the DOC. DOC pressure drop was about 1.4 kPa, and DOC pressure drop at CDPF was about 4.1 kPa. Besides, no significant influence of DOC catalyst composition of CDPF pressure drop under full load was found.