Instant active dry baker’s yeast is a well known product widely used for leaven ing of bread. The yeast produced by fermentation is usually dried by hot air to 94-96% dry matter content. Nowadays, several drying methods are used in industry, in which multi-stage fluidized bed drying process is considered as an effective method for the yeast drying. Temperature and retention time in each stage significantly affect the final activity of the yeast. High temperatures should be avoided during drying in order to maintain the activity. In this study, optimal operating parameters of an industrial continuous fluidized-bed drier for production of instant active dry yeast were investigated. The operating conditions, such as temperature, loading rate of compressed yeast granules, and hot air humidity had direct effects on the yeast activity and viability of the product. The dryer had 4 zone separated with moving weirs. The most important factor that affected the quality of the product was the operational temperature in each zone on the bed. Optimization was performed for three loading rates of the feed to the dryer, using Taguchi method for the experimental design. The samples were withdrawn from each zone of the dryer after reaching steady state conditions, and analyzed with respect to the fermentation activity that is the leavening ability of CO 2 production of yeast cells in dough (by CO 2 evolution method using a fermentograph recorder), viability (by methylene blue method), and the dry substance. The highest fermentation activity for 300, 350, and 400 kg/h loading rate were found to be 620, 655, and 640 Cm 3 CO 2 /h, respectively. The average activity value obtained at 630 Cm 3 CO 2 /h, where the average viability of the cells was 67,74 and 71% at the end of the dryer. The results showed that if the operational temperature of zones 1, 2, 3, and 4 controlled at 33, 31, 31, and 29 °C, the product reach the best activity of 650 Cm 3 CO 2 / h and the highest viability of 76%.