A study of temperature profiles in mixing zone of AHU(air handling unit) can contribute greatly to enhance performance of AHU system, so the study on the temperature distribution between RA(return air) and OA(outdoor air) is important to analyze the mixing characteristics in a mixing zone of AHU. Accordingly, the temperature profiles during RA(return air) and OA(outdoor air) supply process into mixing zone of AHU with an air mixer are studied numerically. The effect of air mixer, OA temperature and RA/OA flow rate are studied in detail. In this study, the results show that the mixing effect is all high for installed the air mixer. The more OA flow rate decrease, the more mixing effect is high.