Background: Carabelli’s trait is an inner knob near the second molar and the first permanent molar and has been used as an important ethnic index for decades. It can be used to show great ethnic differences in dentistry. Objectives: The aim of this longitudinal study was to determine: (1) the frequencies of Carabelli trait and (2) the relationship of Carabelli’s trait between the deciduous and permanent teeth. Method: The sample consisted of 64 pairs of dental casts (32 boys, 32 girls, aged 3–5 and 12–14) on Vietnamese people. Results: The frequencies of pits and grooves were high (deciduous teeth: 51.56%, permanent teeth: 39.06%). By contrast, the frequencies of pronounced tubercular forms were low (25% in both dentitions). The correlation coefficient between the deciduous and permanent teeth was 0.65 (p<0.01). Conclusions: The frequencies of the pit and groove forms of Carabelli’s trait were high in both dentitions. A highly significant positive correlation was noted for Carabelli’s trait between dentitions.