Abstract: Diatom sedimentation is an important carbon sink in the ocean, and its rate affects the efficiency of the biological pump. To understand the effects of diatom shell and cell inclusion on the sinking rate of diatom, the sinking rate of live cell, heat killed cell (containing diatom shells and intact cell inclusions but without activity) and diatom shell of 17 common diatom species in Beibu Gulf were measured using SETCOL method during the exponential phase. The results showed that the sinking rate of live diatom, heat killed cells and silica shells were positively linearly correlated with their size (P<0.01). The role of diatom shells and cell inclusions in determining sinking rate was different according to the range of size. The sinking rate of <25 µm diatoms was mainly determined by its shell and the sinking rate of 25 µm to 65 µm diatoms were influenced by both their shell and inclusions. While the sinking rate of large diatom such as Palmeria hardmaniana, was mainly determined by cell inclusion. Generally, the effects of diatom shells on sinking rate decreased with increasing cell size; in contrast, the effects of cell inclusions increased with increasing size. Most of the diatoms, which sinking rate was dominated by their shell, have an ability to connect with other cells by their surface protrusions; however, the species, which sinking rate was dominated by their inclusions, are independent and usually have a large size and flexible in sinking regulation.