This present study is to determine the potential of Trichoderma harzianum as a biological control agent against phytopathogens. It has been tested to see how well it works against different plant pathogens, including S. rolfsii, Alternaria sp., P. aphenidermatum, M. phaseolina, F. oxysporum, H. sativum, and A. niger. Compared to the other Trichoderma species used, TVK1 has the best ability to kill phytopathogens, with a value of 1.78 cm per day. TVK2 comes in second with 1.7 cm per day, and TVK5 comes in third with 1.74 cm per day. The antifungal volatile organic compound produced by TVK1 spores was higher than the other species examined; it prevents phytopathogens’ growth from the other side. The number of pathogens that T. harzainum can kill is 100% for S. rolfsii, 94.07% for Alternaria sp., 92.02% for A. niger, 100% for F. oxysporum, 100% for H. sativum, and 94.07% for M. phaseolina. In a detached leaf assay, TVK1 inhibited the growth of pathogens in different plant leaves like tomato, mustard, and brinjal when compared with the commercially available fungicide and the control. It is confirmed in this study that the isolate TVK1 can fight against different phytopathogens. The chance of phyto-pathogenic attack can be reduced if sprayed on the plants earlier, as we know that Trichoderma harzianum also acts as a growth promoter.