This paper presents the three dimensional CFD analysis of temperature and velocity profiles of air in an HVAC mixing box. The 3-D CFD simulations and field measurements of temperature stratification [1] in mixing box are compared in this paper. A mixing box is the section of an air handling unit used to mix the return air flow with the outside air flow. Return air is supplied from the top of the box and outside air supplied from the back of mixing box. A three dimensional CFD set of simulations for four different outside air temperatures is done by using Star CD. A k-? turbulence model for high Reynolds number flows is also used. Mixing effectiveness in this box is studied by plotting temperature and velocity profiles using commercially available CFD software. The measurements as well as the simulations show that the temperature difference between the upper part and the lower part of the duct downstream of the mixing box is considerable. It increases, as the outside temperature decreases. However, the discrepancies between the measurements and the 2-D simulations are large in [1]. The reasons for this are uncertain boundary conditions and modeling errors leading to an inaccurate simulation result [1]. Using though the range of measurement accuracy provided in [1] in the current simulation and comparing it with the current simulated results has significantly reduced the discrepancy between simulation and experiment and indicates that the CFD approach can be used to provide quantitative agreement between the two approaches.