This study aims to determine the effects of alfalfa supplementation on the pre- and postweaning performance, rumen development, and feed transition in starter diet-fed lambs. Six of 66 male Hu lambs were slaughtered at the age of 10 d to serve as a control. The other 60 lambs were randomly allocated to 2 dietary treatments: milk replacer and starter pellets without (STA) or with free-choice chopped alfalfa (S-ALF). The animals were offered 300 g/d of the concentrate mixture and had free access to alfalfa after weaning at the end of wk 4 (age 38 d). The alfalfa inclusion in the S-ALF group tended to increase the starter intake before weaning, significantly increased the concentrate intake soon after weaning ( < 0.05), and increased the BW ( < 0.01) and ADG ( < 0.10) in pre- and postweaning lambs. The S-ALF group had heavier carcasses ( < 0.05), rumens ( < 0.05), reticula ( < 0.05), omasums ( < 0.10), abomasums ( < 0.05), and visceral organs ( < 0.10) than the STA lambs after weaning. Alfalfa supplementation increased ( < 0.05) the rumen papillae length and the ratio of the duodenal villus height to the crypt depth; it also decreased ( < 0.05) the concentration and molar proportion of propionate in wk 1 and 5. The STA lambs had higher ( < 0.01) blood concentrations of globulin and blood urea nitrogen and lower β-hydroxybutyrate after weaning. The STA group also had a higher incidence of feed plaque. From the above results, we infer that the free-choice addition of chopped alfalfa to starter diets is beneficial to rumen development, relieves weaning stress, and improves the performance of lambs.