<p> Purpose: After root canal treatment, the tooth is restored with an abutment, for which core build-up resins providing adhesiveness have come into use. This study examined the effect of the thickness ratio of the composite resin used for construction and the thickness of the root dentin on the strength of the construction.</p><p> Methods: This research used four core build-up resins: 1) Clearfil DC Core Automix ONE (DC), 2) BeautiCore Post Paste (BC), 3) Filtek Fill and Core Flowable Restorative Plus (FT), and 4) Estecore (EC). First, 164 experimental specimens of 2.0 mm×2.0 mm×14.0 mm, with dentin thicknesses of 0.5 (D0.5), 1.0 (D1.0), 1.5 (D1.5), and 2.0 mm (D2.0), were prepared from bovine teeth, which were subjected to a three-point bending test in a universal testing machine, and the fracture load was recorded. Second, 40 extracted human single-root teeth were used to perform compressive strength tests of dentin-resin composites. The root side of the tooth was cut to a length of 15 mm from the root apex, and root dentin with the post space of 1/3 width (1/3 P), which is a standard post formation, was used as the control group. Root dentin widths of 2/3 (2/3 P), 3/4 (3/4 P), and 4/5 (4/5 P), and human extracted teeth without core build-up (S) were used as the experimental groups. A universal testing machine was used to apply a static load in the tooth axial direction at a crosshead speed of 1.0 mm/min, and the fracture load was recorded.</p><p> Results: The bending strength of FT was significantly lower than those of BC and EC in the D0.5 group (p<0.05). The bending strength of FT was significantly lower than that of EC in the D1.0 and D1.5 groups (p<0.05).</p><p> The bending strengths of the four core build-up resin groups were significantly greater than that of the S group, which only underwent root canal shaping.</p><p> The highest force required for fracture among the four groups was 41.1±10.6 MPa in the 3/4 P group. This was followed by 39.1±8.8 MPa in the 1/3 P group, 37.5±9.5 MPa in the 4/5 P group, 33.4±4.1 MPa in the 2/3 P group, and 15.3±7.3 MPa in the S group.</p><p> Conclusion: It is suggested that construction using a core build-up resin offers similar compressive strength as conventional abutment construction based on 1/3 width, even if the remaining tooth root is thinner.</p>