Terpenoid phytoalexins are secondary metabolites of plants act as defensive agents against plant pathogen attack. Alternaria cerealis AUMC 14484 (MT808477) is a phytopathogen of tomato that stimulates new types of phytoalexins especially during the biological control process. Trichoderma harzianum utilized as controlling agent of A. cerealis using the fungal filtrates or spore suspension in infected plants. Time course detection of terpenoid phytoalexins was 2, 24 and 48 h of infection and estimated with complete analysis of their types and concentrations using GC-MS analysis. The plant cells behavior during the biological control process was monitored by lipid peroxidation, hydrogen peroxide, and antioxidant enzymes (catalase and peroxidase) analyses. The results showed significant increase in total terpenoids, lipid peroxidation and hydrogen peroxide after 24 h in all treatments; however catalase and peroxidase increased in infected tomato plants but decreased during the biological control process which reflects decrease in the cell stress during the infection. Twelve terpenoid phytoalexins recorded in infected tomato leaves, seven of them are newly recorded in tomato plants including 5à-spirostan-23-ol, (22s,23r,25r); 2(1h)-naphthalenone, octahydro-1-methyl-1-(2-propenyl), (1à,4aá,8Aà); 2,2' (1h,1'h)-spirobi-s-indacene, ethanone.; Spiro[5hbenzocycloheptene 5,1'[2,5] cyclohexadiene] 4',9diol,6,7,8,9 tetrahydro2,3,3',4,5 'pentamethoxy-7,8-dimethyl-, 9-acetate; carvacrol; maslinic acid; Spirost8en11one,3 hydroxy, (3á,5à,14á,20á,22á,25R) and Olean-12-ene-3,15,16,21,22,28-hexol, (3á,15à,16à,21á,22à). Rishitin derivatives (rishitinol and rishitinone) also recorded in infected tomato leaves. The application of Trichoderma harzianum as culture filtrate or spore suspension throughout the biological control procedure is critical for tomato plant resistance against A. cerealis leaf spot disease by enhancing redox buffer capacity, improving plant tolerance, and activating plant defense systems.