Background/Aim: In this study, a new method to create a customized three-dimensional (3D) bolus by accurately considering the anatomy of an individual patient is demonstrated. Patients and Methods: A 3D bolus structure was created from an extended planning target volume (PTV) to reduce an inevitable skin reaction. In addition, during computed tomography simulation in patients with oral cavity cancers, a balloon was inserted into the mouth of a patient to secure space, and then the area surrounding the balloon was designed into a 3D bolus structure. Results: For patients with head and neck cancers, a customized 3D bolus can reduce the unnecessary skin dose by 14.4% compared to a commercial bolus. For patients with oral cavity cancer, the PTV and tongue doses were 93.8% and 8% of the prescribed dose, respectively. Conclusion: The customized 3D bolus enables effective skin sparing and full coverage of the target area.