Although biomechanical properties of dentin are important factors to dentin bonding, as well as for understanding caries, cervical erosion/abfraction, and tooth fracture, limited information for primary teeth has been reported. This study evaluated the micro-tensile strength (MTS) of sound primary second molar dentin with an originally designed system that we have developed. Twenty-seven dumbbell-shaped specimens were prepared from eight teeth. The MTS of the dentin beneath the occlusal surface was measured and fractured dentin surfaces were observed using SEM. Data was analyzed using ANOVA subsequent to Fisher's PLSD at p < 0.05. The novel jig system used in this study allowed symmetric dumbbell-shaped and uniformly sized specimens. The mean (standard deviation) MTS of all the specimens was 38.2 (15.9) MPa. The mean MTS of the specimens sectioned from the central area (46.5 MPa) was significantly higher than those of the specimens that were sectioned from the most mesial (31.1 MPa) and distal (27.8 MPa) sides of the teeth. Sound primary second molar occlusal dentin showed regional variations in tensile strength. This might influence the prognosis of dental restorations.