Tomato (Solanum lycopersicum) is economically the second most valuable vegetable. Tomato fruits are exposed to postharvest microbial decay both in the farm and storage. One of the major diseases of this plant, which annually destroys large amounts of tomato crops, is spotted spot (Alternaria spp.). The pathogen was isolated and purified from suspicious samples to contamination. After studies of the morphological and according to identify key fungal genus Alternaria spp. was known. The most prominent feature of this genus is the production of a chain of dark spores with transverse and longitudinal bands. Pathogenic studies proved by the implementation of Koch's principles. The aim of this study was to investigate the inhibitory activity of hydrogen peroxide (H2O2) on Alternaria spp. fungi in vitro. For this purpose, concentrations (0.2.0.1, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, and 1.5%) of hydrogen peroxide were added to the PDA medium. Petri Dishes containing different concentrations of hydrogen peroxide Inoculated with mycelium discs from young cultivars of pathogenic fungus and stored at 25 ± 2 ° C until the fungus colony filled the petri dish of the control. The fungal growth on the basis of colony diameter was measured by a digital caliper devices. The experiment was carried out in three replications in a factorial design. The results showed that 0.6% to 96% and concentrations of 0.7, 0.8, 0.9 and 1% hydrogen peroxide completely inhibited fungal growth. According to the results, with the determination of the level of control of the Alternaria spp. by hydrogen peroxide, as well as its does not remain on the fruit, the adverse effect on non-target organisms and the risk of human health, preserves as much as possible the freshness and marketability of the product. Further studies on the use of hydrogen peroxide as an alternative to harmful chemical compounds is required.