Abstract: Histone methyltransferase G9a is responsible for catalyzing the mono-, di- and slowly trimethylation of histone H3 lysine 9 (H3K9). It can also specifcally methylate lysine 373 in tumor suppressor p53. G9a plays crucial roles in diverse biological processes and various human diseases especially in the pathogenesis and progress of cancer. It is considered as a promising novel antineoplastic target. Hence, the development of its inhibitors has received increased attention. The research advances in different classes of small molecular inhibitors of G9a reported over the past decade were reviewed, so as to provide reference for the clinical development of small molecular inhibitors and the discovery of novel inhibitors.