The main function of mobile air conditioning system in a vehicle is to provide the thermal comfort to the occupants sitting inside the vehicle at all environmental conditions. Passenger thermal comfort is mainly influenced by the inside cabin temperature and airflow flows through the duct system. The function of ducts is to get the sufficient airflow from the HVAC system and distribute the airflow evenly throughout the cabin. The main focus for all OEMs is to optimize the design of the duct system and satisfy the target requirements such as airflow, velocity and temperature. In this paper, the focus is to optimize the rear passenger floor duct system to meet the target requirements through design for six sigma (DFSS) methodology. Normally floor duct design is evaluated by the target airflow, velocity and temperature achieved at passenger leg locations. Computational fluid dynamics analysis (CFD) has been used extensively to optimize system performance and shorten the product development time. Multiple flow simulation needs to be carried out for various design configurations of floor duct through analysis until the targets are achieved before finalizing the design and its time consuming. In this paper, studies are carried out for creating the parametric modelling and the three dimensional Navier- Stokes equations are solved to determine the pressure drop, airflow and temperature of the floor duct system. Design for six sigma methodology is followed for robust optimization and arrive at the best combination of design parameters which influences the velocity and temperature at passenger leg locations. L12 orthogonal design array matrix has been created and the analysis has been carried out individually to check the velocity, temperature and pressure drop. The impacts of each design parameters and levels have been analyzed extensively and best combination of design parameters have been found out for the rear floor duct to meet the target requirements. Physical testing is carried out for the optimized floor duct design to measure the airflow, velocity and temperature at passenger leg locations. There is a good correlation agreement between simulation and test results for the optimized design. Parametric modelling of floor duct significantly aids in reducing the manual design time for simulation by 40% and the DFSS approach helps in finding out the optimized design parameters of floor duct during the design phase of new programs. This methodology can be followed for optimization of duct systems to shorten the product development cycle of the program.