Thirty weaned male lambs (28 days old) with live weight of 10.3 kg were randomly assigned to one of 3 treatments for a 56 days feeding period to study effects of inoculation with rumen fluid from mature sheep on growth performance and rumen fermentation. Treatments consisted of: (1) starter grain ration (SGR, control), (2) fed SGR and inoculated with 100 ml fresh rumen fluid (FRF) daily for 7 days and (3) fed SGR and inoculated with 100 ml lyophilized rumen fluid (LRF) for 7 days. Results showed that there were no differences in DM intake, apparent digestibilities of crude protein and acid detergent fiber (ADF), ruminal pH, and ruminal concentrations of ammonia N and total volatile fatty acid (VFA). However, both inoculations decreased feed conversion rate (FCR) (P<0.01). Inoculation of FRF increased average daily gain (ADG), apparent digestibilities of DM (P<0.01) and neutral detergent fiber (NDF) (P<0.05), while inoculation of LRF increased apparent digestibility of fat (P<0.05). There were significant interactions between treatment and sampling time for all individual ruminal VFA (P<0.05), except butyric acid and the ratio of acetic to propionic acid (P<0.001). In conclusion, FRF inoculation was beneficial to improving growth performance of lambs during the transition. Further research is needed to explain the mechanism of action of the FRF as probiotic.