Aim: Aims of this research to study the effect of surface area on retention of complete denture. Materials and methods: Oral examination clinically, some properties of the physical and chemical of human unstimulated saliva have been evaluation. They constituted in three groups (25 in each the first and the third group) and (50 in the second group). The first group was healthy and aged (20-30) years, the second group was healthy and aged (50-70) years, also the third group aged (50-70) years but with systemic diseases. The following properties were studied: flow rate, pH, total protein, density, surface tension, viscosity and film thickness. The amount of total protein concentration was responsible for physical and chemical changes especially viscosity. There was a direct positive correlation between the amount of total protein and surface tension, viscosity, density and film thickness, while there was a negative correlation between total protein and with flow rate and pH. The measurement of force due to surface tension of these samples have been done by preparing two pairs of circular discs of (PMMA) of (5.2,6.5) cm in diameter and compare of force between these groups, also compare these physical analyses with mathematic analysis. Results: indicated that in both the first and the third groups, the physical and mathematic analysis for the retention showed there was non-significant changes while second group showed there was a significant changes. Conclusion: there was a positive correlation between force and wetted area and this force was due to the wettability and not to water sorption.