This paper investigates the combustion and emission characteristics of dual fuel combustion mode using port injection of biogas combined with direct injection of commercial diesel oil based on a high pressure common rail diesel engine. The effects of EGR rates on combustion and emission characteristics are explored and results from using different port -injected test biogas including Biogas 1# (H-2:CO:CH4:N-2 = 5:40:5:50) and Biogas 2# (H-2:CO:CH4:N-2 = 15:30:5:50) are also compared. The experimental results show that with the increase of EGR rates, combustion phase retards and ignition delay gets prolonged. NOx emissions decrease monotonically coupled with an effective inhibition once the EGR rate grows over 50%. In the particle size distribution of dual fuel combustion fueled with biogas/diesel, the concentration of nuclear mode particle is high especially for Biogas 1# in high EGR rate. The indicated thermal efficiencies drop gradually as EGR rate increases. It is revealed that under approximately the same EGR level, Biogas 2# with higher hydrogen proportion has higher reactivity when mixed with diesel. With port injection of Biogas 2#, NOx emissions are much higher as well as higher indicated thermal efficiencies are achieved. The high efficiency and low emissions of biogas/diesel dual fuel combustion mode will be achieved under the condition of using EGR, split injection and addition of hydrogen in biogas. (C) 2016 Elsevier Ltd. All rights reserved.