Canola meal (Brassica napus L.) and oriental mustard bran (Brassica juncea) were subjected to alkali and acid pretreatment to expose pentosan, for enhancing further enzymatic hydrolysis by endo-1,4-β-xylanase from Trichoderma longibrachiatum for the production of oligosaccharides. Pretreatment especially with alkali, effectively increased the relative content of pentosan to about ~ 41% and ~ 72%. Alkali pretreated canola meal and mustard bran resulted in a pentose content of 2.28 ± 0.15 g and 3.20 ± 0.11 g per 100 g substrates at 18 h and 24 h of reaction respectively, which corresponded to ~ 26% and ~ 28% conversion of original pentosan in substrates. UPLC-MS data showed xyloglucuronic acid (XGlcA) as the major oligosaccharide in the hydrolyzates. Reversed-phase HPLC-DAD indicated the principal phenolic compound in the hydrolyzates was sinapine. DPPH radical scavenging assay showed that endoxylananse hydrolyzates of acid pretreated substrates had strong antioxidant activities in comparison to alkali pretreated samples.