Abstract The effect of rotary desiccant wheel on the thermal performance of the solar dryer unit is numerically investigated. The solar dryer unit integrated with rotary desiccant wheel consists of rotary desiccant wheel, solar air collector, and drying unit. The theoretical models of the desiccant wheel and solar collector have been validated using an experimental data. Also, the effects of desiccant wheel rotation speed on the performance of this system are investigated. The numerical results of this study show that; for using rotary desiccant wheel in the solar drying units, dry and hot air are produced that, in turn, improve the drying process. Also, for using rotary desiccant wheel in the solar drying units, the temperature of drying air increased from 65 °C to 82 °C while the humidity ratio decreased from 15 to 8.8 g water /kg dry air compared to the solar drying units without using a desiccant wheel, at the same ambient conditions. Furthermore, the results show that the optimal rotation speed of the desiccant wheel which about 15 rph to obtained the maximum drying air temperature and minimum humidity ratio inlet to dryer unit. The percentage increase in the system useful heat gain for using the solar drying unit integrated with rotary desiccant wheel about 153% in average compared to the solar drying unit without rotary desiccant wheel. The advantages of using rotary desiccant wheel in a solar drying unit introduces a continuous drying along the daily time, an increase the rate of drying due to dry and hot air out from rotary desiccant wheel, an increase the quality of the dry products, and a decrease in the time required for drying the product.