Proper flow distribution on the windshield and side windows is critical for adequate visibility while driving. Fog or ice which forms on the windshield is the main reason of invisibility and leads to major safety issue. It has been shown that proper clear visibility for the windshield could be obtained with a better flow pattern and uniform flow distribution in the defrost mode of the automobile heating, ventilation, and air-conditioning (HVAC) system.In this study, a three dimensional numerical model of a car cabin with full HVAC system was developed using Star-CCM+, a commercial CFD package. The Reynolds-Averaged Navier-Stokes equations (RANS) approach with the realizable two-layer k-ε turbulence model was employed for simulating the airflow field on the windshield for the defrost mode. The HVAC unit, ducts and defroster grille were included in the analysis in detail and the air distribution on the windshield was studied. The results of CFD simulations were compared to the experimental data using a correlation study and the correlation coefficient of numerical and experimental measurements was reported. CFD simulation was in good agreement with the experimental study.In order to study the effect of excluding HVAC unit from computational domain a second model was generated and it was found that the inclusion of the HVAC unit in addition to the defrost ducts significantly affects the numerical results. Also, it was shown that, data correlation coefficient along with the average percentage of error study is a reliable tool for comparison of CFD predictions of airflow distribution with experimental study over the windshield surface.