Polymeric dental aligners manufactured by thermoforming are recently used in clinical treatment of misaligned teeth. In this study, a polyester sheet with 0.75 mm thickness was used and the deformability of thermoformed aligner is discussed experimentally with the help of digital image correlation method. The experimental apparatus to apply prescribed displacement to aligners mounted on a teeth model was developed focusing on the situation of treatment for misaligned left canine. The deformation mode was discussed through measured displacement field and maximum principal strain distribution. It was found that mechanical behavior of aligner was influenced more significantly by the shape of teeth, movement vector of left canine and crooked shape of aligner especially at inter-teeth borders than the thickness distribution of thermoformed aligner. The role of deformable edge part of the aligner was also revealed.