An engineered beta-mannanase (mRmMan5A) from Rhizomucor miehei was successfully expressed in Pichia pastoris. Through high cell density fermentation, the expression level of mRmMan5A reached 79,680 U mL(-1). The mRmMan5A showed maximum activity at pH 4.5 and 65 degrees C, and exhibited high specific activities towards mannans. To produce manno-oligosaccharides, palm kernel cake (PKC) was pretreated by steam explosion at 200 degrees C for 7.5 min, and then hydrolyzed by mRmMan5A. As a result, the total manno-oligosaccharide yield reached 34.8 g/100 g dry PKC, indicating that 80.6% of total mannan in PKC was hydrolyzed. Moreover, the kilo-scale production of manno-oligosaccharides was carried out to verify the feasibility of mass production. A total of 261.3 g manno-oligosaccharides were produced from 1.0 kg of dry PKC. An effective beta-mannanase for the bioconversion of mannan-rich biomasses and an efficient method for the production of manno-oligosaccharides from PKC are provided in this paper.