Abstract: In order to investigate the effects of particle size and content of calcium carbonate on the micropore structure of cigarette paper in the vicinity of char line, mercury porosimeter and scanning electron microscope(SEM) characterization methods were established to find out the relationship between the CO delivery in mainstream cigarette smoke and the change of micropore structure of cigarette paper in the vicinity of char line during burning. The results showed that: 1) Either increasing the particle size or content of calcium carbonate promoted the micropore amount of cigarette paper in the vicinity of char line and uniform distribution of pore size. 2) The CO delivery in mainstream cigarette smoke decreased with the increase of the total pore volume of roasted cigarette paper, especially the increase of volume of pores of 0.1-8.0 μm in size. The increase of micropore amount in the vicinity of char line facilitated CO diffusion and air dilution, as a results, decreased CO delivery in mainstream cigarette smoke by 0.7-1.0mg/cig.