The transformation system mediated by Agrobacterium tumefaciens is a kind of common and efficient means for gene transformation of dicotyledons. The advantages of this system have aroused much attention. This method is used recently to transform monocotyledons which have been considered out of the range of Agrobacteria hosts. Many important crops such as rice, corn, barley and wheat have been transformed successfully by Agrobacterium. This article summarizes the advantages of Agrobacteria transformation, the mechanism of transformation and research advances on monocotyledons.