The effect of air drying temperature on shrinkage, rehydration ratio and microstructure of dried onion slices (3±0.2 mm thickness and 30 mm diameter) was studied. The different drying conditions led to distinctive structural changes in the samples, affecting the shrinkage and rehydration ratio. Experiments showed that the intensity of structural changes depends on temperature and drying times. A higher temperature and time caused greater damage to the microstructure of onion slices, resulting in the formation of a highly porous structure. The results also indicated that increasing time and temperature increased the shrinkage and rehydration ratio of samples. In mathematical modeling of shrinkage, the model proposed in this study, which had R2 values higher than 0.978 and SSE values pretty low, predicted accurately the experimental data.