Abstract: The resistance of 4 alfalfa (Medicago sativa) cultivars to pea aphid (Acyrthosiphon pisum) was evaluated by massinfesting young seedlings in the field and lab to select high resistant cultivars to pea aphid. The results of this study showed that the Gannong No.5（HA3）showed the strongest resistance and its percent of resistant plants accounted for 50.3% and 48.9% in the field and lab experiments, respectively. The Hunter River showed a poor resistance and its percent of resistant plant was only 4.0% and 4.3% in the field and lab experiments, respectively. The field experimental results showed that Gannong No.5 was high resistance cultivar, and Gannong No.3, Golden Empress and Hunter River were susceptible cultivars; the lab experimental results showed that Gannong No.5 was high resistance cultivar, and Gannong No.3 and Golden Empress were low resistance cultivars, and Hunter River was susceptible cultivar.