Abstract: We studied the colonization of the biocontrol agent, bacterium SW11, in tomato and its rhizosphere soil by using the antibiotic labeling. Results showed that strain SW11 colonized in tomato root, stem, leaves, and rhizosphere soil. Population was the highest at 5 days after applying the bacteria in tomato plants. At 30 days after applying the bacteria, population of SW11 was 1.31×10⁴ cfu/g at root surface, 4.88×10² cfu/g in rhizosphere soil, 1.84×10² cfu/g in leaf surface, 0.87×10² cfu/g in stem outside, 20—65 cfu/g in the root, stalk and leaf inside. Bioassay indicated that fermentation liquid of the bacteria significantly suppressed grey mold disease caused by Botrytis cinerea. It prevented the disease by 88.15% at 7 days after application at 1.57×10⁸ cfu/mL. This research showed that strain SW11 is of great potential to be a biocontrol agents in suppressing tomato grey mould disease.