At present, Computational-Fluid-Dynamics (CFD) with the ＇standard＇ k-epsilon model is a popular method for numerical simulation of room airflow. The k-epsilon model needs a lot of computing time and large a computer. This paper proposes a new zero-equation model to simulate three-dimensional distributions of air velocity, temperature, and contaminant concentrations in rooms. The method assumes turbulent viscosity to be a function of length-scale and local mean velocity. The new model has been used to predict natural convection, forced convection, mixed convection, and displacement ventilation in a room. The results agree reasonably with experimental data and the results obtained by the standard k-epsilon model. The zero-equation model uses much less computer memory and the computing speed is at least 10 times faster, compared with the k-epsilon model. The grid size can often be reduced so that the computing time needed for a three-dimensional case can be a few minutes on a PC.