Gene encoding for β-mannanase (E.C 3.2.1.78) from Klebsiella oxytoca KUB-CW2-3 was cloned and expressed by an E. coli system resulting in 400 times higher mannanase activities than the wild type. A 3314bp DNA fragment obtained revealed an open reading frame of 1164bp, namely kman-2, which encoded for 387 amino acids with an estimated molecular weight of 43.2kDa. It belonged to the glycosyl hydrolase family 26 (GH26) exhibited low similarity of 50-71% to β-mannanase produced by other microbial sources. Interestingly, the enzyme had a broad range of substrate specificity of homopolymer of ivory nut mannan (6%), carboxymethyl cellulose (30.6%) and avicel (5%), and heteropolymer of konjac glucomannan (100%), locust bean gum (92.6%) and copra meal (non-defatted 5.3% and defatted 7%) which would be necessary for in vivo feed digestion. The optimum temperature and pH were 30-50°C and 4-6, respectively. The enzyme was still highly active over a low temperature range of 10-40°C and over a wide pH range of 4-10. The hydrolysates of konjac glucomannan (H-KGM), locust bean gum (H-LBG) and defatted copra meal (H-DCM) composed of compounds which were different in their molecular weight range from mannobiose to mannohexaose and unknown oligosaccharides indicating the endo action of mannanase. Both H-DCM and H-LBG enhanced the growth of lactic acid bacteria and some pathogens except Escherichia coli E010 with a specific growth rate of 0.36-0.83h(-1). H-LBG was more specific to 3 species of Weissella confusa JCM 1093, Lactobacillus reuteri KUB-AC5, Lb salivarius KL-D4 and E. coli E010 while both H-KGM and H-DCM were to Lb. reuteri KUB-AC5 and Lb. johnsonii KUNN19-2. Based on the nucleotide sequence of kman-2 containing two open reading frames of 1 and 2at 5' end of the +1 and +43, respectively, removal of the first open reading frame provided the recombinant clone E. coli KMAN-3 resulting in the mature protein of mannanase composing of 345 amino acid residues confirmed by 3D structure analysis and amino acid sequence at N-terminal namely KMAN (GenBank accession number KM100456). It exhibited 10 times higher extracellular and periplasmic total activities of 17,600 and 14,800 units than E. coli KMAN-2. With its low similarity to mannanases previously proposed, wide range of homo- and hetero-polysaccharide specificity, negative effect to E. coli and most importance of high production, it would be proposed as a novel mannanase source for application in the future.