The aim of this study was to compare bone formation and tissue reactions around implants with a calcium phosphate (CaP)-coated machined surface (CPMS) and a CaP-coated rough surface (CPRS) at critical-sized circumferential ridge defects in dogs after 4 and 8 weeks.All mandibular premolars were extracted bilaterally from five male mongrel dogs. After 8 weeks, circumferential marginal defects (2 mm wide and 5 mm deep) were created unilaterally using a customized drill at sites of CPMS and CPRS implant insertion. Four weeks later, the contralateral side was prepared using the same protocol. The dogs were sacrificed 4 weeks later. Specimens were analyzed histologically and histomorphometrically.Both CPMS and CPRS implants showed successful osseointegration in the apical bone area (without defects) regardless of the observation period (4 or 8 weeks). All circumferential defects were incompletely filled and osseointegration was limited at the apical area of the defect. In the unfilled defect area, collagen fibers were arranged perpendicular or oblique to the exposed implant surfaces, especially on CPRS implants.Within the limits of this study, CPMS and CPRS implants showed no statistically significant differences in osseointegration and bone regeneration in circumferential defects, along with the unique feature of collagen fibers arranged perpendicular or oblique to the exposed implant surfaces.