Introduction: Sexual dimorphism refers to differences in size, stature and appearance between male and female. It is a known fact that tooth crown is formed to full size in childhood even before eruption into oral cavity. The shape and size of the teeth permits an interesting dimension of study for sexual dimorphism, we aimed to delineate the sexual dimorphism by measuring the mesiodistal (MD) and buccolingual (BL) diameters of permanent maxillary first molar in Nepalese population of Eastern Nepal. Objectives: The aim of this study was to evaluate the sexual dimorphism in maxillary first molar among Nepalese population of eastern Nepal Methodology: 100 participants of either sex (50 males & 50 females) aged between 17 to 25 years were enrolled in the study. After thorough dental examination, impression of the maxillary arch was made and MD diameter and BL diameters were measured with the help of vernier caliper. A P value of <0.05 was considered as statistically significant. Results: Sexual dimorphism was found in MD and BL diameters of maxillary first molar in males measuring higher than females. Right mesiodistal showed the highest sexual dimorphism, whereas right buccolingual showed the least. Conclusions: This study confirms that permanent maxillary first molar shows significant sexual dimorphism, out of which mesiodistal measurement stands out to be the best parameter in Nepalese population of Eastern region.