Abstract: Alfalfa（Medicago sativa） is an important forage plants.In order to effectively express foreign genes in alfalfa,by using the green fluorescent protein（GFP） as report gene and Nicotiana benthamiana as a comparison,the effects of two different vectors（the non-replicating and the BeYDV（bean yellow dwarf virus）-based replicating vectors）,and two different Agrobacterium tumefaciens strains（LBA4404 and EHA105） on the transient expression of alfalfa leaves were investigated.The results showed that the combination of LBA4404with the replicating vector（re-vector/LBA4404） led to effective expression of GFP in alfalfa leaves.By using LBA4404 with the replicating vector,the effects of the concentration of re-vector/LBA4404 and incubation time after infection on the transient expression were also evaluated.It was found that with the increase of concentration of bacterial suspension or time post-infection,the expression level of GFP increased first but then decreased.In the leaves of alfalfa,the highest expression level of GFP was observed when the concentration of LBA4404 strain was 1.0(OD₆₀₀);and the expression level of GFP reached the peak at 5-7 d after infection.Compared with Nicotiana benthamiana,achieving the highest expression level of GFP in alfalfa might need higher concentration of Agrobacterium tumefaciens and longer incubation time after infection.