Purpose: This study attempted to (1) clarify the factors influencing the clinical composite assessment of denture-supporting tissues in edentulous patients by experienced prosthodontists and (2) develop a method for quantitative assessment of supporting tissue. Materials and Methods: Seven three- or four-grade scales for clinical factors were used to evaluate supporting tissue in 317 complete denture patients. The general assessment of supporting tissue was conducted with a 10-point scale. The contribution of each grade of the seven factors to the general assessment level was determined by a multiple regression analysis. Results: Based on the contributions of the seven factors, a method for quantitative assessment of supporting tissue was established, category scores for these factors were calculated, and the sums of the category scores were converted to an integer between 0 and 100. The resulting quantification score was closely correlated with the general assessment of the supporting tissue by experienced prosthodontists. Conclusion: The factors influencing the clinical composite assessment of denture-supporting tissues in edentulous patients by experienced prosthodontists were clarified. The quantitative assessment method developed in this study is simple and might be used for preoperative diagnosis and treatment planning.