Operating room ventilation is quite important to protect patients from infection risk inside a surgical field. The laminar flow units (LAF) used in these applications contribute to removal of pathogen particles from the operation area by creating a protection zone on the surgical field. In this study, effect of LAF diffuse size on airflow and particle distribution in an operating room is investigated using computational fluid dynamics (CFD). Three different diffuser sizes (1.8 m x 2.4 m, 2.4 m x 2.4 m, 3.2 m x 3.2 m) and three different particle diameters (5 mu m, 10 mu m, 20 mu m) are examined. In the modeling of airflow and particle motion, the Realizable k-epsilon turbulence model and Lagrangian approach are used, respectively. The distributions of airflow and particles originated from the surgical team are analysed. Also, the air change efficiency is predicted as for the entire operating room and for the breathing zone. The results reveal that the LAF diffuser size has a decisive role in air velocity and contaminant distribution in the operating room. It is seen that the amount of particles deposited on the operating table could be reduced up to 73% by increasing the diffuser size.