Abstract: In order to investigate the resistance mechanism of tobacco varieties to black shank, resistant varieties (Florida 301, L8, Beinhart1000-1 and Gexin 3) and susceptible varieties (Zhongyan 100, Guiyan 13, Honghuadajinyuan and Xiaohuangjin 1025) were selected to assess the effects of root exudates from eight tobacco varieties on the mycelium growth of Phytophthora parasitica var. nicotianae (Breda de Hean) Tuker. The results showed that the root exudates from the four resistant varieties had clear inhibitory effects on the mycelium growth of P. parasitica var. nicotianae with inhibition ratios from 5.93% to 20.78%. The colony diameters treated by root exudates from the four resistant varieties were significantly smaller than the control (P < 0.05). Comparing with the control, the root exudates from susceptible varieties (Guiyan13, Honghuadajinyuan and Xiaohuangjin 1025) promoted the mycelium growth of P. parasitica var. nicotianae with the inhibitory ratios from -16.47% to -2.15%. The colony diameters treated by root exudates from the three susceptible varieties were extremely significantly larger than the control (P < 0.01). However, susceptible variety Zhongyan 100 did not promote the mycelium growth of P. parasitica var. nicotianae. Therefore, the effects of tobacco root exudates on the mycelium growth of P. parasitica var. nicotianae were correlated to the resistance of tobacco varieties.