More than 90 different modified nucleosides have been identified in tRNA. Among the tRNA modifications, the 7-methylguanosine (m⁷G) modification is found widely in eubacteria, eukaryotes, and a few archaea. In most cases, the m⁷G modification occurs at position 46 in the variable region and is a product of tRNA (m⁷G46) methyltransferase. The m⁷G46 modification forms a tertiary base pair with C13-G22, and stabilizes the tRNA structure. Recently, we have proposed a reaction mechanism for eubacterial tRNA m⁷G methyltransferase (TrmB) based on the results of biochemical studies and previous biochemical, bioinformatic, and structural studies by others. However, an experimentally determined mechanism of methyl-transfer remains to be ascertained. The physiological functions of m⁷G46 in tRNA have started to be determined over the past decade. To be able to better respond to diseases and infections in which the m⁷G modification is considered to be involved, it is still necessary to further understand the catalytic mechanism of AdoMet and/or the tRNA bound form of m⁷G methyltransferases. In this review, information of tRNA m⁷G modifications and tRNA m⁷G methyltransferases are summarized and the differences in reaction mechanism between tRNA m⁷G methyltransferase and rRNA or mRNA m⁷G methylation enzyme are discussed.