The planes which serve as references for cranium and face in dental clinical application included the occlusal plane, Frankfort plane, Camper's plane and hamular–incisive–papilla (HIP) plane. The HIP occlusal plane is a horizontal plane passing through the bilateral hamular notches and the incisive papilla (Dent Surv. 1975;51:60). The aim of this study was to estimate the relationship between the various occlusal planes and the HIP plane in Taiwanese young adults with approximately optimal occlusion. Study casts of 100 young adults (50 men and 50 women) were selected in this study. All market points on the maxillary casts were measured by a three-dimensional precise measuring device. The angular relationship between the four various occlusal planes and the HIP plane were investigated. The vertical distances between the cusp tips and incisal edges of maxillary teeth to the HIP plane were measured. Data were performed by the Statistic analysis software programme (JMP 4.02). The Student's t-test and Pearson's correlation test were used to test the statistical significance (P < 0·05). The results showed that the occlusal plane defined as the incisal edge of maxillary central incisor to mesiobuccal cusp tips of maxillary second molars had the smallest included angle with the HIP plane (2·61 ± 0·81°). The incisal edge of maxillary right central incisal to mesiopalatal cusp tips of maxillary first molars had the largest included angle with the HIP plane (7·72 ± 1·60°). The curve is drawn through the buccal cusp tips of maxillary teeth had better parallelism with the HIP plane.