Endophytic Bacillus subtilis Y10 isolated from Chinese cabbage (Brassica rapa) can control effectively cruciferous vegetable clubroot disease. To understand its clubrootcontrol mechanisms, GFP plasmids were transformed into Y10 cells for fluorescencetagging by natural transformation. The plasmid stability, growth curve and plate antagonism bioassay of GFPtagged strains were determined, and the results showed only plasmid pHT01P43GFPmut3a could be maintained stably and its introduction had no significant negative influence on Y10 growth and antagonistic performance. Pot bioassay experiment with different cultivation matrixes indicated that there was no significant difference in clubrootcontrol efficiency between Y10P43GFPmut3a and its wild type. Recovery and observation by confocal laser scanning microscope confirmed that, 9 days after inoculation, tagged strain could colonize the root, stem and leaf of Chinese cabbage, and the colonization density still kept at 10³-10⁴ cfu·g^-1 tissues in 44 days after inoculation. All the results suggest that the clubrootcontrol of Y10 be intimately associated with efficient colonization in Chinese cabbage host.