The aim of this study was to determine the optimal heating conditions for sheet forming using a circle tray by comparing the thickness and fit of mouthguards formed under different conditions. Mouthguards were fabricated using ethylene vinyl acetate sheets (4.0mm thick) and a vacuum forming machine. The working model was trimmed to a height of 20mm at the incisor and 15 mm at the first molar. Two forming conditions were compared: square sheets were pinched by the clamping frame attached to the forming machine; and round sheets were pinched at the top and bottom and stabilized by a circle tray. Each condition was defined when the sheet sagged by 10-mm or 15-mm below the level of the clamp. The thickness of the sheet was determined for the incisal and molar portion. Additionally, the difference in fit according to the forming conditions was measured by examining the cross section. Differences in the thickness or the fit due to forming conditions were analyzed using two-way analysis of variance (ANOVA) followed by Bonferroni's multiple comparison tests. The thickness after formation was thicker at the 10-mm condition than that of 15-mm, and the fit at the 15-mm condition was better when that of 10-mm with square and round sheets. Within the limitation of this study, it was suggested that when forming a mouthguard using a 4.0-mm EVA sheet and a circle tray on a vacuum forming machine, the sheet should be formed at a sagging distance of 10-mm.