Phoma stem canker or blackleg that caused by Leptosphaeria maculans, is the most destructive and important disease of canola (Brassica napus) in the world. A study was conducted to investigation of trichoderma inhibitory effect on Leptosphaeria maculans the causal agent of canola black leg that isolated from Golestan, Gorgan, Mazandaran, Sari, Behshahr, Noshahr and Qazvin at greenhouse conditions in three stages of Hayola (One of the canola cultivars) growth stage such as seed, double leaf stage and 4-8 leaf stage of canola. Actually based on the biology of pathogen, the samples was collected from early of spring to lately of summer from canola field in three stages such as Rosette stage for collection of anamorph form, leafs with symptom from reproductive phase until the harvest and final stage from Herbal Remnants for determine of telemorph form. Based on the Laing and Deacon method, pathogen hyperparasitism test was performed in laboratory conditions. Effect and relationship of fungi was investigated under microscope. Based on microscopic observation complexity, hyphal penetration of trichoderma into and around of pathogenic fungi, hyphal fragmentation of pathogenic fungi was seen. In greenhouse conditions, suspensions with dilution about 108 espore of isolates from trichodermia and pathogenic pathogens in three replications were performed in randomized completely block design and then data were analyzed by SPSS software. Based on result of seed treatment of canola in greenhouse condition, T39 and T20 of tichoderma isolate respectively was reduced pathogenicity about 65.2% and 58.6% but in shoot treatment with trichoderma isolates respectively was reduced Pathogenicity about 65.2% and 58.6%. Actually based on result, trichoderma isolate can have a significant effect on prevention of disease when use them in pre-planting.