In this study, the effect of optimum fly ash addition on porosity and compressive strength of concrete is investigated. A factorial analysis approach is applied to find out the main influencing factors on the chosen properties. In the analysis, three input factors were used, such as total binder content, water/binder ratio and fly ash/binder ratio as independent variables. The compressive strength and mercury intrusion porosimetry (MIP) tests were conducted on the specimens at the age of 90 days. Test results were analyzed and evaluated by using ANOVA (Analysis of Variance). Statistical models are derived for each measured concrete property in two curing conditions (in air and in water) by means of three independent components of concrete. Influence of selected mixture parameters on concrete properties was evaluated by using the test results. Various optimum fly ash contents were determined for each concrete property according to the derived models and discussed thoroughly.