Environmental changes such as global warming can influence the level of disease caused by a pathogen through affecting the host – pathogen interaction. Little studies have been carried out on studying the effect of temperature increase on plant disease resistance. There is evidence that the temperature affects the stability of both quantitative and qualitative resistance to Leptosphaeria maculans. L. maculans is the cause of blackleg (BL) or phoma stem canker on a number of members of Brassicaceae, including canola (Brassica napus). There are two types of symptoms that can be seen from the BL disease: leaf spot and stem canker. Regarding the impact of global warming on BL disease and resistance in canola, the present study was designed to investigate the effect of increased temperature on host resistance gene to the BL pathogen. The study was laid out as a factorial experiment with a completely randomized design with two factors: canola cultivar (the susceptible Drakkar cultivar without any resistance gene and the resistant Excel cultivar, carrying Rlm7) and temperature (optimum and high temperature regimes 21/16 and 28/23ºC) each with three replicates. Cotyledons of experimental plants were inoculated with 107 spores/ml suspensions and water (control) by drop-wound method, respectively. 14 days after intubation, the lesions were digitally imaged and the areas of spots were measured using ImageJ software by importing snapshots taken from symptoms to a desktop computer. Inspection of these symptoms for 30 plants showed that in the susceptible cultivar spot sizes dropped twice at high temperature compared to the inoculated plants incubated at optimum temperature. On the other hand, the size of spots on the resistant cultivar increased three times at high temperature in comparison with those incubated at optimum temperature. Shrinkage of the size of spots on the susceptible cultivar can be due to the reduction of the virulence of the pathogen, increased physiological vigor of the plant such as leaf area, and also closure of the stomata which leads to a decrease penetration of the pathogen. In addition, the effect of temperature on the resistance gene and its reduced efficiency can be mentioned as the possible causes of the increased spot size at high temperatures relative to optimum temperatures. Despite the reduced ability of the fungus and improved plant vigor, susceptibility of the host plant was observed possibly due to suppression of resistant pathways at high temperature in the resistant cultivar. Since the genetic background plays a key role in single Rlm gene-mediated resistance expression, this study clarified that Rlm7 resistance gene specific for L. maculans in Excel cultivar was temperature sensitive.