BACKGROUND Post-translational modifications play key roles in various biological processes. Protein arginine methyltransferases (PRMTs) transfer the methyl group to specific arginine residues. Both PRMT1 and PRMT6 have emerges as crucial factors in the development and progression of multiple cancer types. We posit that PRMT1 and PRMT6 might interplay directly or in-directly in multiple ways accounting for shared disease phenotypes. OBJECTIVE To investigate the mechanism of the interaction between PRMT1 and PRMT6. METHODS Gel electrophoresis autoradiography was performed to test the methyltranferase activity of PRMTs and characterize the kinetics parameters of PRMTs. Liquid chromatography-tandem mass spectrometryanalysis was performed to detect the PRMT6 methylation sites. RESULTS In this study we investigated the interaction between PRMT1 and PRMT6, and PRMT6 was shown to be a novel substrate of PRMT1. We identified specific arginine residues of PRMT6 that are methylated by PRMT1, with R106 being the major methylation site. Combined biochemical and cellular data showed that PRMT1 downregulates the enzymatic activity of PRMT6 in histone H3 methylation. CONCLUSIONS PRMT6 is methylated by PRMT1 and R106 is a major methylation site induced by PRMT1. PRMT1 methylation suppresses the activity of PRMT6.