Endo-β–mannanases in plant require post-translational modification, such as N-glycosylation and disulfide-linked dimerization, for their catalytic activity. Determination of the plant endo-β–mannanase activity needs to modify the assay conditions for optimizing their enzymatic reaction. Here, we describe a modified method for plant endo-β–mannanase assay. A high-salt buffer without thiol reductants is required for effective extraction of the enzyme. The enzyme is able to digest water-insoluble AZCL galactomannan to release water soluble dyed fragments, which is detected through measurement of absorbance at 590 nm wavelength. Increase in absorbance at 590 nm is correlated directly with enzyme activity.