This study was performed to estimate the exchange of air (ventilation) and temperature distribution in the cylinder that simulated human body. Simulation simplified the human body wearing the running wear jacket with slits. Slits were positioned at the shoulder, mid-back and lower-back. For the running wear jacket, non-air permeable material was assumed to eliminate the effect of porosity of the fabrics. Airflow and microclimate temperature were analyzed using computational fluid dynamics (CFD). The results showed that the air tended to rise and drift towards the slits. Air flown out through the slits was in the order of the lower-back slit > mid-back slit > shoulder slit. Discrepancy in the air flow rate at each slit was caused by the generation of ascending air currents according to slit positions. The direction of the air current depended on the microclimate temperature inside the jacket. These results indicate that the lower-back slit gives better air exchange effect which was in agreement with the clothing microclimate observed by human wear tests.