Abstract: A Chlamydomonas reinhardtii immobile mutant named ift81 was screened by insertional mutagenesis. The mutant was bald or with two short flagella. Gene sequencing confirmed that fifty-two base pairs in the fifth extron of IFT81 gene were replaced by the exogenous gene aphⅧ in the mutant. After a recombinant plasmid containing full IFT81 gene was imported into the mutant ift81,the phenotype of the mutant was rescued and the expression of IFT81-HA could be detected,which confirmed that the defect in IFT81 gene led to the abnormal flagella directly. Electron microscopy images exhibited the abnormal flagella of the mutant. An immunofluorescence assay revealed that IFT81 protein was localized to the basal body and flagella. This work shows that loss of protein IFT81 will directly lead to the defects of flagellar assembly,and IFT81 protein is indispensable for flagellar assembly in Chlamydomonas reinhardtii.