This paper presents a detailed experimental study on the particulate matter (PM) emission characteristics in reactivity controlled compression ignition (RCCI) mode on a modified single cylinder diesel engine, which combines port injection of biogas with in-cylinder direct injection of diesel. In this experiment, two types of four-component syngas have been selected as test fuels to simulate 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). The PM emission characteristics are analyzed in detail by varying the premixed ratio, the overall lower heating values (LHVs) per-cycle and the main direct injection timing. The results demonstrate that the particles emitted from biogas/ diesel in the RCCI combustion mode are primarily nucleation mode particles. With the increase of the premixed ratio, the particulate number and particulate surface area concentration increase significantly, and the particle geometric mean diameter (GMD) also gradually increases, as well. The particulate number and the volume concentration show notable increase with the advance of the main direct injection timing. The peak values of the particulate number size distributions shift in the direction of smaller sizes with higher proportion of H2 in biogas; thus, the particle GMD is slightly reduced.