RNA methylation, which was identified back in 1970s, has gained remarkable interest in recent years as it was shown to be a reversible modification involved in many cellular processes like mRNA and miRNA processing, mRNA localisation, translation suppression, or activation. These, in turn, affect important bioprocesses such as tissue development, sex determination, and DNA damage response. Important group of proteins are responsible for adding, recognizing, and removing the methyl group to and from the RNA molecules, which are referred as writers, readers, and erasers, respectively. If any of the processes is not strictly controlled, this can cause abnormalities in gene expression, which result in diseases including cancers such as lung, pancreas, glioblastoma, and breast cancer. Mechanisms of RNA methylation and its role in various cancer types and diagnostic methods for RNA methylation are discussed in this article.