Research interest on fiber has increased rapidly in the last decade due to its complexity and functional properties towards the human body. In this research group, pitaya (Hylocereus polyrhiuzus [F.A.C. Weber] Britton & Rose) stem flour is a promising candidate as new food resources since it contains a high amount of fiber and vitamin C as well as demonstrates both antioxidant and antibacterial activities. However, this pitaya stem flour has very low solubility in water that makes its application in food product very limited. In this study, pitaya stem flour has been fermented by Trichoderma reesei in a variation of inoculum concentration (3 % and 5 %) and fermentation time (24 to 60 h). The fermentation processes reduce the ash and water content of the flour, increase the soluble fiber content as well as its solubility. During the fermentation, the colour of the flour has changed. It becomes brighter and brownish. The best fermentation condition marked by its highest soluble fiber content is achieved by 3 % (w/v) of T. reesei inoculum with 60 h incubation time.