Properties of air are interrelated in a drying process. Equations of psychrometrics and fluid dynamics developed by previous researchers enable the development of simulation models, which become an indispensable tool for the study of drying technology. A Windows based software, entitled SAPGD (Simulation of Air Properties and Grain Drying), written in MATLAB, was developed to simulate the equations/models related to the following four aspects: 1. The equilibrium moisture content of grains under the diverse conditions of atmosphere. 2. The pressure drop to airflow of various grains. 3. The relationship between drying time and moisture content under the model of thin layer drying of grains. 4. The relationships among drying time, moisture content, grain depth, and grain temperature under the logarithmic model of deep bed drying of grains.