The aim of this study was to investigate the thickness of mouthguard sheet after vacuum-pressure formation based on the mouthguard sheet material. Three mouthguard sheet materials (4.0 mm thick) were compared: ethylene-vinyl acetate co-polymer (EVA), olefin co-polymer (OL), and polyolefin-polystyrene co-polymer (OS). The working model was made by hard gypsum that was trimmed to the height of 20 mm at the cutting edge of the maxillary central incisor and 15 mm at the mesiobuccal cusp of the maxillary first molar. Where the center of the softened sheet sagged 15 mm lower than the clamp, the sheet was pressed against the working model, followed by vacuum forming for 10 s and compression molding for 2 min. The thickness of mouthguard sheets after fabrication was determined for the incisal portion (incisal edge and labial surface) and molar portion (cusp and buccal surface), and dimensional measurements were obtained using a measuring device. Differences in the change in thickness due to sheet materials were analyzed by one-way analysis of variance (ANOVA) followed by Bonferroni's multiple comparison tests. The OL sheet was thickest at all measurement points. At the incisal edge and cusp, thickness after formation was highest for OL, then EVA and finally OS. At the labial surface and buccal surface, the thickness after formation was highest for OL, then OS and finally EVA. This study suggested that post-fabrication mouthguard thickness differed according to sheet material, with the olefin co-polymer sheet having the smallest thickness reduction.