Abstract: We isolated the endophytic Bacillus amyloliquefaciens YTB1407 from the root of Panax quinquefolium, which has both biological control and growth promoting effects. To investigate its potential applications, a pot experiment of sweet potato was tested to assess the capacity of endophytic colonization of YTB1407 and the selection of its optimum concentration by investigating the performance of root characteristics on three time points in the whole early growth phase after irrigating with different concentrations of bacterial suspensions with treatment of sterile water as control. The activities of endogenous hormone IAA, ZR, t-ZR and IAA oxidase (IAAO, PPO, POD) were analyzed. The results showed that YTB1407 promoted the specific colonization of root system, the elongation of adventitious root and branch roots, and root activity in the early growth stage of sweet potato. At later growth stage, it formed greater fresh mass of absorption root and lower aboveground/root system mass ratio. YTB1407 suspensions with OD₆₀₀ of 0.50 (T_0.50) had more significant effect, which induced the highest fresh tuber mass and the largest effective tuber numbers of per plant at top cover stage. YTB1407 promoted the differentiation of adventitious roots into tubers at initial point of tuberization by increasing IAA content and the ratio of (t-ZR+ZR)/IAA, decreasing IAAO activity and enhancing PPO activity. Moreover, it promoted the differentiated roots into tubers at tuberization stage by keeping the higher content of IAA, lower ratio of (t-ZR+ZR)/IAA, and decreasing IAAO and PPO activities.