N6-methyladenosine (m6A), the most abundant internal modification of RNA in eukaryotic cells, has gained increasing attention in recent years. The m6A modification affects multiple aspects of RNA metabolism, ranging from RNA processing, nuclear export, RNA translation to decay. Emerging evidence suggests that m6A methylation plays a critical role in cancer through various mechanisms. Moreover, m6A methylation has provided more possibilities for the early diagnosis and treatment of cancers. In this review, we focus on m6A-associated mechanisms and functions in several major malignancies and summarize the dual role of m6A methylation as well as its prospects in cancer.