This experiment was conducted to study the biological characteristics and effects of pre-fermentation microcapsulated Lactococcus lactiss on growth performance of broilers. The Lactococcus lactiss strain was selected by the growth performance, lactic acid yield, inhibition of pathogen and resistance to adversity, and identified by Biolog automatic micro-analysis system and 16S rRNA gene. Then, a total of 180 one-day-old AA broilers were randomly assigned to three groups (the control group, broth group， microcapsules group) with 6 replicates per group and 10 broilers per replicate. The experiment lasted for 42 d. The results showed as follows: by Biolog automatic micro-analysis system and 16S rRNA gene comparing with GenBank, the strain GJ0007 was primarily identified as Lactococcus lactis ss lactis. Other experiments on biological characteristics showed that GJ0007 has fast growth （time per generation<30 min）, short lag phase, high L-lactic yield （5.6 g/L）, and high tolerance of reverse environment (including H2O2, artificial gastric juice and intestinal juice), and it can inhibit pathogenic bacteria (E-coli K88 and K99, Staphylococcus aures, Salmonnella pullorum, and Pasteurella sp.), etc. During the whole feeding period (1 to 42 d), comparing with the control group and broth group, average daily gain (ADG) of broilers fed with microcapsulated GJ0007 was increased by 16.94% (P<0.05) and 11.25% (P<0.05), while the ratio of feed to gain (F/G) was decreased by 16.13% (P<0.05) and 619% (P>0.05) , and the relative weight of thymus and bursa of Fabricius of 42-day-old broilers was increased by 46.56% （P<0.05, thymus）, 18.38% （P<0.05, thymus）, 16.67% （P<0.05, bursa of Fabricius） and 5.66% （P>0.05, bursa of Fabricius）. These results indicate that microcapsulated Lactococcus lactis can increase ADG and decrease F/G, and increase the relative weight of thymus and bursa of Fabricius of broilers.［Chinese Journal of Animal Nutrition，2011，23（4）：631 -639］