Abstract: The morphological structure and chemical composition of wheat straw stems and leaves can significantly affect the enzymatic conversion of the carbohydrate. This paper investigated the effect of sodium carbonate(SC)pretreatment on the structural components and substrate enzymatic digestibility(SED)of wheat straw stems and leaves. Structures of stem and leaf lignin before and after SC pretreatment were characterized by wet chemistry methods. Compared with stems, wheat straw leaves showed better delignification effect in the SC pretreatment. The SED of SC-pretreated leaves(TTA content of 8%, 140 ℃)reached 80.5% when hydrolyzed at an enzyme loading of 10 μmol/(min·g), which was 29% higher than that of stems. The results from nitrobenzene oxidation(NBO)and ozonation indicated that the structure of leaf lignin was different from that of the stem with respect to smaller syringyl units and less β-O-4 linkages. After the SC pretreatment, the reduction of the product yield in the NBO and ozonation of leaf lignin were higher than those of stem lignin, showing that more β-O-4 bonds in leaf lignin were cleaved in the pretreatment. It could be deduced that the different SED between wheat straw leaves and stems correlated with the lignin structure to some degrees. It was confirmed that the content and structure of lignin are important factors affecting the enzymatic saccharification of lignocellulosic biomasses.