Abstract : The use of clear orthodontic aligners has increased as an alternative to conventional braces. Therefore, knowledge about the characteristics of the aligner materials can help to provide more accurate treatment results and give orthodontists who are utilizing or planning to employ this technology more information about their behaviour. This study aimed to investigate and compare the effect of thermocycling on the retention measurement of two aligner products used in the fabrication of orthodontic aligners. Two thermoplastic products of Duran® 0.75 mm and Erkodur® 1 mm were used in this study. Digital impressions were obtained for the upper and lower arches of 10 patients. Digital models with two rectangular attachments in the first and second premolars were constructed by using blue sky plan software. After the construction of orthodontic aligners, retention tests were measured using a tensile test with a universal testing machine to measure the maximum forces required to remove the aligners from the models. Twenty aligners were fabricated for each type of product, and measurements were conducted before and after thermocycling. Duran® upper aligners retention showed no significant changes in comparison with Erkodur®, however, Duran® lower aligners showed significant changes in comparison with Erkodur® before thermocycling. However, Duran® aligners showed no significant change, whereas, Erkodur® in both upper and lower arches showed a significant decrease in retention after thermocycling. The retention of the Duran® aligner is higher than that of Erkodur® and thermocycling decreased the retention of the aligners fabricated from Erkodur® in comparison with Duran® aligners.