In order to determine the colonization ability of Bacillus subtilis L1-21 in Citrus leaves and fruits and its control effect on fruit green mold,the colonization dynamics and downwardconduction of B. subtilis in citrus leaves were investigated by foliar spraying with 106 cfu/mL green fluorescent labelled strain (L1-21GFP),colonization in citrus fruits was checked through spraying and soaking with 108 cfu/mL L1-21GFP,and control effect on green mold was evaluated by spore injection with Penicillium digitatum and incidence investigation. After treatment for 10 min,the colonization of B. subtilis L1-21GFP in leaf surface and mesophyll reached 1.36×103 and 8.08×102 cfu/cm2,respectively,and displayed a continuous increasing trend. At 1 h after treatment,the GFPtagged endophyte was stable in leaf surface with 1.51×102 cfu/cm2. The bacterial concentration reached 5.67 cfu/cm2 in mesophyll at 2 h after treatment,which gradually increased to 1.15×103 cfu/cm2 after 30 h. Endophyte L1-21GFP could easily colonize in fruits,and bacterial concentration in pulp and peel after 1 h was 9.51×103 cfu/g and 3.51×104 cfu/g,respectively. However,there was significant difference in the colonization ability in fruits among different citrus varieties,and Citrus reticulata cv Ponkan had the highest colonization. In addition,after soaking for 30 min,the control effect of L1-21 on fruit green mood was 100% on the third day,and 80.36% on the fifth day. Taken together,B. subtilis L121 could colonize in citrus leaves and fruits effectively,providing a theoretical basis for further application of this strain to control citrus diseases.