Background and Objective: After orthodontic treatment, it is very important to maintain and preserve them and prevent them from returning to their original position. Given that the lifespan of different thermoplastic orthodontic aligners affects the durability of thermoplastic retainers, this study was conducted to compare the mechanical characteristics of three types of thermoplastic retainer materials before thermoforming (BT) and after thermoforming (AT). 
Methods: In this in-vitro study, 120 sheets of three types of materials were used: Clear Advantage Series I, Clear Advantage Series II, and Leone, each material having a thickness of 1 mm. The elastic modulus and surface hardness were measured and compared for 60 thermoformed and 60 non-thermoformed samples. Each material was measured in two states of BT and AT. 
Findings: The investigated materials' elastic modulus differed significantly BT (G1: 2172.13±64.78, G2: 2394.47±113.40, G3: 2557.69±159.90). In addition, it differed significantly AT (G1: 2062.97±66.79, G2: 1834.47±73.67, G3: 2432.79±71.52). All studied materials showed a significant decrease in elastic modulus AT (p≤0.05). In the case of surface hardness, it differed significantly BT (G1: 9.815±0.465, G2: 7.915±0.251, G3: 10.948±0.873). In addition, it differed significantly AT (G1: 11.185±0.387, G2: 12.035±0.261, G3: 11.498±0.381). All studied materials showed a significant increase in hardness AT (p≤0.05). 
Conclusion: The results of this study showed that thermoforming can greatly affect thermoplastic materials' mechanical characteristics, making them less flexible and harder. In addition, these findings showed that PETG would be an appropriate choice for applications requiring a high degree of stiffness and that polypropylene could be more suitable for applications requiring a high degree of hardness making them more abrasion resistance. Copolyester does not have the most outstanding values in every characteristic, which offers average properties regarding stiffness and hardness.