Using of dryer accompanied by heat pump with a view to ability of this type of dryer in reduction energy consume and preserving the quality of dried products, specially in case of agricultural products, has considerably developed. For designing new dryers, the moisture content and temperature of grain should be predicted at different location of the drying chamber at any moment. The cheapest and time saving method to predict the drying process is mathematical modeling and computer simulation of the process. In the present study drying of paddy rice in a fixed deep bed grain dryer accompanied by heat pump has been simulated. In order to do this simulation, at first the mathematical model of paddy rice drying process in a fixed deep bed grain dryer was developed and the set of coupled partial differential equations was solved simultaneously by using the backward implicit method. The validity of the model was evaluated by comparison between calculated parameters and experimental results published in the literature for the same system. Then, to complete the work heat pump mathematical model was developed and coupled with the dryer model to have a unit model for heat pump dryer. This model was solved by a computer program in MATLAB 7.0 environment. In order to evaluate performance of heat pump dryer specific moisture extraction rate (SMER) was calculated for the heat pump dryer as well as a dryer with electrical heater. Average values of SMER calculated for heat pump dryer and electrical dryer were 0.4513 kg/kWh and 0.2266 kg/kWh, respectively. These results showed considerable improvement in heat pump dryer efficiency respect to conventional dryer. Finally to reduce the moisture gradient that develops during drying which is one of the important problems in fixed-bed dryers, the idea of reversing the airflow direction was proposed. Optimum time of reversing of airflow direction was calculated and drying process with reversing airflow direction was simulated. The results showed that by applying this method maximum deviation of grain moisture from the ultimate favorite moisture can be reduced from 2.43% to 0.58% and thus the quality of the dried paddy rice is improved.