Erwinia amylovora bacterium is the cause of fire blight disease and is one of the most important damage factors in pome fruit orchards. Considering the bacterial resistance to copper compounds and antibiotics, use of natural antimicrobial compounds has recently been proposed to control plant diseases, such as fire blight. The present study aimed to evaluate the antimicrobial effects of Trans-Cinnamaldehyde, which is a natural compound found in the essential oil of some herbs, on fire blight in the rootstocks of Pyrus communis L. cv. Harrow Sweet cultivated under in-vitro conditions. Herbal samples belonged to the Harrow Sweet variety, which is semi-resistant to fire blight. Average rootstocks used in the study were 2-3 centimeters and the substrate was prepared on a QL culture medium base containing 1 mg/L of Benzylaminopurine (BAP) and 0.1 mg/L of Naphthaleneacetic acid (NAA), with 30 grams of Sucrose, 11 grams of Agar, and 0.5 gram of Pectin. Following that, a Trans-Cinnamaldehyde concentration was obtained to inhibit the phytotoxic effects of the rootstocks. To infect the rootstocks, the Ea273 isolate was used as the inoculum, and the ends of the rootstocks were inoculated with 108 cfu/ml concentrations. Changes in the rootstocks were evaluated at 24-hour intervals for nine days. In addition, the incidence of symptoms was assessed in terms of the final development of necrosis, percentage of the necrotic tissues to the length of the whole branch, and necrosis of the internodes. According to the results, the progression of necrosis in the Harrow Sweet cultivar was faster and more frequent compared to the control samples treated by Trans-cinnamaldehyde. Symptoms of necrosis in the Harrow Sweet cultivar treated by Trans-Cinnamaldehyde were observed after 144 hours, while tissue necrosis in the rootstocks was observed after 72 hours in the rootstocks. The results of the study also indicated that the necrosis of the internodes occurred after 144 hours, while the symptoms were observed after 72 hours in the control rootstocks. Therefore, it could be concluded that Trans-Cinnamaldehyde has proper antimicrobial effects on controlling fire blight. Furthermore, pear rootstocks cultivated under in-vitro conditions could be used to evaluate the effectiveness of antimicrobial agents in controlling the disease as an efficient method for greenhouse practices.