Underfloor air distribution (UFAD) system is widely used in commercial buildings and data centers for its potential advantages of superior thermal comfort and indoor air quality, etc. In UFAD system, conditioned air is directly supplied to the occupied zone from a pressurized plenum, which causes temperature stratification from the lower zone to the upper zone. The airflow distribution is determined by the pressure decay and flow pattern in the plenum, which is affected by the size of plenum, the open area of the diffusers, air leakage, and the obstructions under raised floor. This paper summarizes the results of an experimental study of airflows through plenum and perforated tiles in a plenum test facility, and the governing equations and boundary conditions on the distribution of velocity and pressure in the plenum are deduced. The linear models of the lost coefficient of air leakage and the velocity ratio of outlet and inlet of the plenum are proposed to simplify the measurement and calculation of the distribution of velocity and pressure in the plenum. The results show that the calculated results coincide with the experimental results, and the models will help to understand the design and operation for the UFAD system. (C) 2014 Elsevier B.V. All rights reserved.