To investigate the early bone formation in beagles with mini-lateral window sinus floor elevation and simultaneous implant placement. Six beagles were selected for the split-mouth design procedures. In each animal, one maxillary recess received a 5mm-diameter mini-round lateral osteotomy (test group), and the contralateral maxillary recess received a large rectangular osteotomy (10mm long and 8mm wide), (control group). Simultaneous implant installation was executed on bilateral maxillary recesses. Tetracycline and calcein dyes were administered on the 14th, 13th days and the 4th, 3rd days prior to sacrifice, respectively. After 8weeks of healing, the beagles were euthanized for fluorescent labeling and histomorphometric analyses. In both groups, new bone formation initiated from the circumferential native bone of the maxillary recesses and extended toward the central sub-recess cavities. The maxillary recesses with the mini-window procedures exhibited superior mineral apposition rate, bone formation rate, and the percentage of new bone area to those of the group exposed to large osteotomy procedure (p<.05). While there was no significant difference in the value of bone-to-implant contact, the mini-window group displayed a tendency for an increase in this aspect (p>.05). Bone formation rate and new bone amount were not statistically correlated with bone-to-implant contact (p>.05). The hypothesis that mini-lateral window sinus floor elevation with simultaneous implant placement would improve early new bone formation in augmented sinus compared with large lateral window procedure is accepted.