N6-methyladenosine (m6A), the most prevalent chemical modification found on eukaryotic mRNA, is associated with almost all stages of mRNA metabolism and influences various human diseases. Recent research has implicated the aberrant regulation of m6A mRNA modification in many human cancers. An increasing number of studies have revealed that dysregulation of m6A-containing gene expression via the abnormal expression of m6A methyltransferases, demethylases, or reader proteins is closely associated with tumorigenicity. Notably, the molecular functions and cellular consequences of m6A mRNA modification often show opposite results depending on the degree of m6A modification in specific mRNA. In this review, we highlight the current progress on the underlying mechanisms of m6A modification in mRNA metabolism, particularly the functions of m6A writers, erasers, and readers in the context of tumorigenesis.