Abstract: To facilitate early diagnosis and identification of tomato wilt disease caused by Fusarium oxysporum, tomato seedlings were cultured in liquid medium, and their vascular bundle browning and antioxidative system response after infection with different concentrations of F. oxysporum were investigated. Five concentrations of F. oxysporum - B1 (10⁴ cfu·mL^-1), B2 (10⁶ cfu·mL^-1), B3 (10⁷ cfu·mL^-1), B4 (10⁸ cfu·mL^-1) and CK (the control without pathogenic bacteria) - were used in the study. The antioxidant system variations and vascular bundle browning were assessed after 4, 8, 12, 16 and 20 days of inoculation. The results showed that under hydroponic conditions, vascular bundle browning occurred at 16 days after inoculation with F. oxysporum. Browning severity was positively correlated with inoculant concentration. Vascular bundle pathogen content was detected only under B4 treatment. Melonaldehyde (MDA) content of tomato leaves initially decreased followed by an increase with the passage of time. It started rising gradually on day 12 and reached the maximum on day 20 after inoculation. MDA contents in inoculation treatments were higher than the control and B4 treatment was significantly higher than the other treatments. Peroxidase (POD) activity initially decreased slowly but picked up 12 days after inoculation, followed by a sharp rise under inoculant concentration of 108 cfu·mL^-1. Polyphenol oxidase (PPO) activity increased gradually until reaching its peak value 16 days after infection. MDA content and activities of POD and PPO increased with increasing inoculant concentration, and were highest at inoculant concentration of 10⁸ cfu·mL^-1, which respectively were 13.1, 12.9 and 1.9 times of the un-inoculated control. No changes were observed in catalase (CAT) activity within the entire culture experiment. CAT activity of control treatment was higher than that of inoculation treatments, and was not affected by F. oxysporum inoculation. When combined with vascular bundle browning, MDA content and POD and PPO activities were important indicators for early diagnosis of tomato Fusarium wilt.