Abstract: In the laboratory and greenhouse studies, 203 strains of Fusarium oxysporum collected from northern China were inoculated onto watermelon plants, and 109 of them were selected as non-pathogenic. These non-pathogenic strains were divided into three groups, i.e. stimulating, suppressing or no effect on the growth of watermelon plants. Ten strains representing the stimulating group, with high colonization frequences in vascular bundles of the plant, were pre-inoculated into the soil media before seeding watermelon in the greenhouse and field plots. The results showed that two strains, strain 316 and 317, (with challenge inoculum rate of 2% ) delayed the occurrence of Fusarium wilt symptoms by 7-10 days. When the challenge inoculum rate was 0.01%, the suppression effect on Fusarium wilt 25 days after the inoculation in the greenhouse tests was 34.6% and 68.2% for Strain 316 and Strain 317, respectively. This suppressing effect improved as the challenge inoculum rate decreased.