In our previous study, we generated a strain of 19-P (1030) in which artificial RNA interference (RNAi) was induced by transcribing a hairpin RNA of ~780-bp stem. We utilized this RNAi-induced strain to uncover RNAi-related genes. Random insertional mutagenesis was performed to generate tag-mutants that show a RNAi deficient phenotype. The 92-12C is one such tag-mutant, which bears a 14-kb deletion in chromosome 1. Complementation of 92-12C revealed that a protein gene, including a Cys-Cys-Cys-His-type zinc finger motif and an ankyrin repeat motif, is essential for effective RNAi in Chlamydomonas reinhardtii (Dangeard). BLAST analysis revealed that the zinc finger protein is homologous to an mRNA splicing-related protein of other species. Therefore, one of the probable scenarios is that mRNA coding for RNAi-related proteins cannot be properly spliced, which causes RNAi deficiency in the 92-12C tag-mutant.