In vitro studies showed significant inhibitory effects of hydrogen peroxide (H2O2) on growth of either sunflower root/collar rot pathogen, Sclerotium rolfsii or the biocontrol agent Trichoderma harzianum. H2O2 provided synergistic effects with T. harzianum, since the antagonistic potentiality of T. harzianum against S. rolfsii was increased with increasing H2O2 concentration. Sunflower root/collar rot severity was significantly reduced when seeds were singly treated by H2O2 or T. harzianum. H2O2 appeared to be the most effective to prevent S. rolfsii infectivity followed by T. harzianum. The present study provided synergistic effects for H2O2 to improve biocontrol capacity since H2O2 at 1.2g/l with T. harzianum achieved sunflower protection by 78% against S. rolfsii infection.