Fire blight, caused by the bacterium Erwinia amylovora, is one of the most destructive diseases of plants in the Rosaceae subfamily pomoideae. The objective of this study was to determine the level of resistance of 30 pear cultivars and describe the biochemical changes induced in some resistant and susceptible cultivars. The level of resistance was determined by the length of the fire blight lesion as a percentage of overall shoot length. Finally, ‘Dargazi’ and ‘Harrow Sweet’ as very resistance cultivars, ‘Bartlett’ as susceptible cultivar and ‘Mohamad Ali’ as a very susceptible cultivar were selected for biochemical evaluation. Some antioxidant enzymes such as ascorbate peroxidase (APX), catalase (CAT), guaiacol peroxidase (POX) and total phenol content were measured at 0 (before inoculation) 3, 6 and 12 days after inoculation by Erwinia, in order to find out their resistance mechanisms when get attacked by the pathogen. Catalase and ascorbate peroxidase activity showed a significant increase in the leaves of very resistance cultivars compared with susceptible cultivars during Erwinia infection with the highest activity in 6^th day after treatment. Guaiacol peroxidase activity was increased in very resistance cultivars survey until 3^rd day after inoculation, but decreased even more than that in susceptible cultivars, in 6^th day. Pathogen attack caused an increase in total phenol content in all infected cultivars, but significant difference was observed among cultivars.