Aim: Ethylene, a phytohormone, is produced during in vitro plant tissue culture and its accumulation is associated with reduction of growth and morphological changes. The aim of this study was to evaluate application of CoCl2, as ethylene synthesis inhibitor, on growth and some physiological parameters as well as expression of ACC oxidase gene of potato.Material and Methods: In this study, nodal segments of potato (Solanum tuberosum L.) cv. White Desiree were cultured on MS medium containing concentration of 0, 5, 10, 15, 20, 30, 40, 60 mg/L CoCl2. After 5 weeks, chlorophyll a, b, total, cartenoied, content, leaf area and expression level of ACC oxidase gene were investigated.Results: CoCl2 inhibited root production in aerial parts and increased leaf areas and amount of chlorophyll a, b and carotenoid. Application of 20 mg/L CoCl2 was the best for potato growth. ACC oxidase gene expression was decreased at this concentration.Conclusion: Application of CoCl2 decreased ACC oxidase gene expression and prevented ethylene accumulation in the in vitro culture and consequently improved growth of potato plant.