Abstract: The cores, thin?section and SEM data were used to research the diagenesis of volcaniclastic rocks and its effects on reservoir quality of Tongbomiao formation in Tanan sag in Inner Mongolia. The results show that the volcaniclastic rocks of Tongbomiao formation mainly consist of ignimbrite, tuff, tuffites, tuffaceous sandstone and tuffaceous conglomerate. In addition to the compaction, cementation and dissolution, the diagenetic characteristics of the volcanoclastic rocks are of peculiar properties, compared with normally sedimentary clastic rocks, such as clinkering process, devitrification and recrystallization. The clinkering and devitrification processes result in abundant secondary pores which improve the physical property of the reservoir. The existence of microcrystalline quartz cladding is beneficial to the conservation of primary intergranular porosity. The compaction and the carbonate cementation are mainly factors to allow the reservoir quality to be poor