RNA N-methyladenosine (mA), the most abundant internal modification of mRNAs, plays key roles in human development and health. Post-translational methylation of proteins is often critical for the dynamic regulation of enzymatic activity. However, the role of methylation of the core methyltransferase METTL3/METTL14 in mA regulation remains elusive. We find by mass spectrometry that METTL14 arginine 255 (R255) is methylated (R255me). Global mRNA mA levels are greatly decreased in METTL14 R255K mutant mouse embryonic stem cells (mESCs). We further find that R255me greatly enhances the interaction of METTL3/METTL14 with WTAP and promotes the binding of the complex to substrate RNA. We show that protein arginine N-methyltransferases 1 (PRMT1) interacts with and methylates METTL14 at R255, and consistent with this, loss of PRMT1 reduces mRNA mA modification globally. Lastly, we find that loss of R255me preferentially affects endoderm differentiation in mESCs. Collectively, our findings show that arginine methylation of METTL14 stabilizes the binding of the mA methyltransferase complex to its substrate RNA, thereby promoting global mA modification and mESC endoderm differentiation. This work highlights the crosstalk between protein methylation and RNA methylation in gene expression.