Purpose: The aim of this study was to evaluate and compare the long-term clinical and radiographic outcomes between a bone graft substitute mixture (beta-tricalcium phosphate [β-TCP] and plateletrich plasma [PRP]) and β-TCP bone graft substitute alone used for sinus floor elevation. Materials and Methods: This randomized clinical trial included patients with an atrophic maxilla referred for maxillary sinus floor elevation. The elevated sinus cavities of patients were randomly filled with β-TCP plus PRP (study group) or β-TCP alone (control group). Residual bone crest height, vertical bone height gain, and bone graft resorption were measured on cone beam computed tomography (CBCT) images at 10 days and 6 months postoperatively. Incidence of sinus membrane perforations and maxillary sinus infections were recorded. Paired t and Student t tests were used for intragroup and intergroup comparisons, respectively. Results: The sample was composed of 18 subjects: nine subjects in the control group (mean age, 31.51 years) and nine subjects in the study group (mean age, 34.01 years). The mean residual bone crest height was found to be 5 mm in both groups (4.88 mm in the control group and 2.70 mm in the study group, with no significant difference). From the 10-day to 6-month postoperative visit, mean vertical bone height gains were changed from 12.48 to 11.59 mm in the study group and from 14.77 to 13.19 mm in the control group, with no significant difference. The mean vertical bone graft resorption was -1.58 mm in the study group and -0.89 mm in the control group, with no significant difference. Sinus membrane perforation was observed in 3 of 18 patients. Conclusion: In this study, PRP plus β-TCP graft substitute did not produce significantly more vertical bone height gain or significantly less vertical bone graft resorption compared with β-TCP graft substitute alone. Within the limitations of this study, however, it can be concluded that both grafting materials produced sufficient vertical bone height gain for safe implant placement. Keywords: beta-tricalcium phosphate, cone beam computed tomography, platelet-rich plasma, sinus floor augmentation