Reactions of 20 cucumber cultivars, tested to Fusarium stem and root rot disease, have ranged from susceptible to tolerant. The most susceptible cultivars were 8-Ayat, 32-PV, CB- 61688222, Janette, Nasco, Negeen, Nicoo 100, PSR36-47112, Rubah-l, Rubah-s and Sultan and tolerant cultivars were 120118, Ayat, Jakie, FD-C101, Festival, Khassib, PSR 36-45007, SR36-45664 and Storm. Between these cultivars, Negeen as susceptible cultivar and Festival as tolerant cultivar were selected for biochemical studies of soluble peroxidase (SPOX) and the roots phenolic compounds. The seedlings with 1-2 true-leaves of each cultivar were transplanted into inoculated soil by two pathogen isolates. Plants treated similarly and transplanted in sterile soil containing sterile mixture of sand and corn meal were used as control. One, three, five, seven and 10 days after inoculation, samples were prepared from crown and roots of infected plants and used for enzyme extraction, and the activities of soluble peroxidase (SPOX) and the roots phenolic contents were determined based on spectrophotometric analysis. Results of experiments on quantitative changes phenolic compounds in tolerant and susceptible cultivars (Festival and Negeen respectively) indicated that there had been no significsnt differences of total phenol content in the infected tissue (mg per gr of fresh weight of root) of tolerant cultivar compared to the susceptible one. Based on results of peroxidase activity, there had been significant differences of activities of soluble peroxidase in the infected tissue of tolerant cultivar compared to the susceptible one, and the content of enzyme in infected tissue of tolerant cultivar was more than the susceptible one.