A family of PRDM proteins are similar to histone methyltransferases (HMTases) with SET domain in that they modulate different cellular processes, including transcriptional regulation, through chromatin modifying activities. By applying a bioinformatic approach, we searched for proteins containing the SET domain and identified a double zinc-finger domain containing PRDM8 with HMTase activity. In vitro HMTase assay and immunoblot analysis revealed that PRDM8 specifically methylates H3K9 of histones which indicates transcriptional repression activity of PRDM8. Direct recruitment of PRDM8 to the promoter mediated transcriptional repression and indicated no involvement of HDAC. Tissue blot analyses identified PRDM8 transcripts from brain and testis in adult mouse. Consistent with these observations, we demonstrate that PRDM8 repressed the expression of steroidogenic markers, p450c17c and LHR, which indicates its regulatory role in mouse testis development.