Statement of problem: The application of elastomeric ligatures is one of the methods of archwires ligation to orthodontic brackets. Knowledge about modulus of elasticity of these ligatures is useful for selecting a specific brand of elastomeric ligature for a special condition.Purpose: The purpose of this study was to compare elastic modulus of seven commercially available elastomeric ligatures in the initial condition and after 28 days immersion in a simulated oral environment. Changes in the modulus of elasticity was also surveyed for each brand of ligatures after 28 days.Materials and Method: In this experimental laboratory-based study, elastomeric ligatures were obtained from different companies and their modulus of elasticity were measured in the initial condition and after 28 days immersion in a simulated oral environment using Zwick testing machine at 5mm/min cross head speed. The data were analyzed using SPSS software and ANOVA, Duncan and student's t-test.Results: Different brands of elastomeric ligatures showed significant differences in elastic modulus both in the as-received condition and after 28 days immersion in the simulated oral environment (p<0.05). Ligatures from American Orthodontics and Ortho Technology showed the lowest and ligatures from Dentaurum revealed the highest elastic modulus in the as-received condition. Modulus of elasticity in the ligatures from American Orthodontics, Ortho Technology and ITO increased significantly while in ligatures from Dentaurum it was decreased significantly  after 28 days immersion in the simulated oral environment (p<0.05).Conclusion: Ligatures from American Orthodontics and Ortho Technology are suitable for immediate use after bonding and in sliding mechanics while ligatures from Dentaurum are not suitable to use immediately after bonding.Key words:Elastomeric ligatures, Elastic Modulus, BondingStatement of problem: The application of elastomeric ligatures is one of the methods of archwires ligation to orthodontic brackets. Knowledge about modulus of elasticity of these ligatures is useful for selecting a specific brand of elastomeric ligature for a special condition.Purpose: The purpose of this study was to compare elastic modulus of seven commercially available elastomeric ligatures in the initial condition and after 28 days immersion in a simulated oral environment. Changes in the modulus of elasticity was also surveyed for each brand of ligatures after 28 days.Materials and Method: In this experimental laboratory-based study, elastomeric ligatures were obtained from different companies and their modulus of elasticity were measured in the initial condition and after 28 days immersion in a simulated oral environment using Zwick testing machine at 5mm/min cross head speed. The data were analyzed using SPSS software and ANOVA, Duncan and student's t-test.Results: Different brands of elastomeric ligatures showed significant differences in elastic modulus both in the as-received condition and after 28 days immersion in the simulated oral environment (p<0.05). Ligatures from American Orthodontics and Ortho Technology showed the lowest and ligatures from Dentaurum revealed the highest elastic modulus in the as-received condition. Modulus of elasticity in the ligatures from American Orthodontics, Ortho Technology and ITO increased significantly while in ligatures from Dentaurum it was decreased significantly  after 28 days immersion in the simulated oral environment (p<0.05).Conclusion: Ligatures from American Orthodontics and Ortho Technology are suitable for immediate use after bonding and in sliding mechanics while ligatures from Dentaurum are not suitable to use immediately after bonding.Key words: Elastomeric ligatures, Elastic Modulus, Bonding