In order to understand the effect of thermoforming on the interlayer adhesion of coextruded films, a peel test was performed for coextruded films after being stretched. To simulate the non-uniaxial stretching nature of actual thermoforming processes, planar stretching and biaxial stretching were applied to the coextruded films prior to the peel test. Both the planar stretching and biaxial stretching were performed at an optimum thermoforming temperature under well-controlled stretch rates to a predetermined stretch ratio. It was found that there was a significant amount of reduction of interlayer adhesion due to stretching. Furthermore, the loss of interlayer adhesion at the optimum thermoforming temperature was linearly related to thickness drawdown as a result of stretching regardless of stretching modes. Therefore, it is suggested that the effect of thermoforming on interlayer adhesion of coextruded films can be easily estimated from the thickness distribution of thermoformed parts.