Signal transduction pathways utilize posttranslational modifications to regulate the activity of their components in a temporal‐spatial and efficient fashion. Arginine methylation is one of the posttranslational modifications that can result in monomethylated‐, asymmetric dimethylated‐ and/or symmetric dimethylated‐arginine residues in proteins. Here we demonstrate that inhibitory‐Smads (Smad6 and Smad7), but not receptor‐regulated‐ (R‐)Smads and the common‐partner Smad4, can be methylated by protein arginine N‐methyltransferase (PRMT)1. Using mass‐spectrometric analysis, we found that PRMT1 dimethylates arginine74 (Arg74) in mouse Smad6. PRMT1 interacts with the N‐terminal domain of Smad6 in which Arg74 residue is located. Assays examined so far have shown no significant differences between the functions of Smad6 and those of methylation‐defective Smad6 (Smad6R74A). Both wild‐type and Smad6R74A were equally efficient in blocking BMP‐induced growth arrest upon their ectopic expression in HS‐72 mouse B‐cell hybridoma cells.