Fusarium oxysporum is a well-known soil-borne fungi and it is difficult to control their pathogenic strains by conventional strategies. The cultures of two strains of Trichoderma harzianum (T16 and T23) were examined in laboratory conditions and with pot experiments for the control of pathogenic strains of Fusarium oxysporum f. sp. melongenae (Fomg), Fusarium oxyporum f. sp. lycopersici (Fol), Fusarium oxysporum f. sp. niveum (Fon) and F. oxysporum f. sp. melonis (Fom). The T16 and T23 strains showed significant inhibition of mycelial growth in the pathogenic strains of F. oxysporum and the maximum inhibition were recorded when the T. harzianum strain T16 was used (72.69%). Both T. harzianum strains produced volatile and non-volatile metabolites that inhibited growth of F. oxysporum strains on PDA medium. In vitro colonization study demonstrated the root-colonizing ability of these antagonists. The interaction between T. harzsianum isolates (T16 and T23) and pathogenic F. oxysporum hyphae showed no overgrowth, hyphal coiling, cell wall degradation or any hyphal penetration around any of the tested F. oxysporum hyphae. Pre-treatment of soil with T16 significantly reduced the severity of Fusarium wilt disease. The disease severity in control plants reached to 90-95% whereas those of the T16-Fomg and T16-Fol treated seedlings of eggplants were 37.74% and 47.12%, respectively, on the 21st day. In this study, while both T. harzianum isolates had a considerable antagonistic effect on the tested pathogens, T16 was found robe more successful than T23. The strong repressive effect of T. harzianum (T16) towards pathogenic Fusarium oxsporum can be applied in biological control of these pathogens.