N-methylguanosine (mG) is a positively charged， essential modification at the 5' cap of eukaryotic mRNA， regulating mRNA export， translation， and splicing. mG also occurs internally within tRNA and rRNA， but its existence and distribution within eukaryotic mRNA remain to be investigated. Here， we show the presence of internal mG sites within mammalian mRNA. We then performed transcriptome-wide profiling of internal mG methylome using mG-MeRIP sequencing (MeRIP-seq). To map this modification at base resolution， we developed a chemical-assisted sequencing approach that selectively converts internal mG sites into abasic sites， inducing misincorporation at these sites during reverse transcription. This base-resolution mG-seq enabled transcriptome-wide mapping of mG in human tRNA and mRNA， revealing distribution features of the internal mG methylome in human cells. We also identified METTL1 as a methyltransferase that installs a subset of mG within mRNA and showed that internal mG methylation could affect mRNA translation.