Axonemal dyneins are essential structures to promote cilia motility and are preassembled in the cytoplasm before being transported into cilia and flagella.hsC21orf59,homolog of Chlamydomonas FBB18,a conserved protein containing a coiled-coil domain and DUF2870 domain,was recently identified as a protein responsible for dynein preassembly in both zebrafish and human.However,it is still unclear how FBB 18 is involved in dyneins preassembly.Here,we showed that FBB 18 is not only responsible for the preassembly of DHC onto outer dynein arms but also participate in assembly of flagella.We obtained a fbb18 null mutant which has very short and various length flagella by insertional mutation in Chlamydomonas.The slightly remained flagella showed partial loss of outer-dynein arms and the remained outer-dynein arms without DHC.By constructing a truncated FBB18 with deletion of the last 25AA,we found that flagella length was completely rescued but not motility.Electromicroscope of truncated FBB18 flagella showed the same structure defects as the null mutant.We conclude that FBB18 is not only involved in cilia motility by promoting the preassembly of DHC onto outer dynein arms and the integrity of DUF2870 domain is especially indispensable for its function in cilia motility but also essential for flagellar assembly.