Orange is one of the most important citrus species in the economy and industry. This study aimed to dry the oranges with a combined pretreatment of osmotic solution and ultrasound waves to reduce the drying time and improve the quality of the dried product. In this study, a hot air dryer was used to investigate and optimize drying time, water reabsorption capacity of dried samples and shrinkage index of dried samples of orange slices. The samples were pretreated by immersing them in an osmotic solution with brix at three levels of 15, 30 and 45 degrees and applying ultrasonic waves to the samples at three-time levels of 10, 20 and 30 min in the ultrasonic bath. The pretreated samples were dried at three temperature levels of 70, 80 and 90℃ and three hot air velocities of 1, 1.5 and 2 m/s. The results showed that by increasing the concentration of an osmotic solution from 15 to 45 and increasing the ultrasonic time from 10 min to 30 min, the total drying time and the shrinkage index of the product decreased. The water reabsorption of the dried samples was dependent on the variables of ultrasonic time and osmotic solution concentration (P<0.01). The best state of water reabsorption, which was 8.9, occurred at 15° Brix and 30 min of ultrasound. The optimization results showed that the optimal point with the highest desirability index (0.808) occurred at 45° Brix of the osmotic solution, 30 min of ultrasound, 70°C and 1.07 m/s of hot air dryer.