Ammonia Fiber Expansion (AFEX) treatment is a technique that is able to enhance the enzymatic hydrolysis yield of lignocellulosic materials. In this technique, lignocellulosic materials are treated by liquid ammonia under pressure followed by rapid release of pressure that expands the fiber structure and increases enzyme access to lignocellulose polysaccharides. However, the AFEX treatment variables such as the mass ratio of ammonia to lignocellulosic biomass, moisture of lignocellulose (moisture content of biomass), temperature, and residence time need to be evaluated to find the maximum efficiency of this treatment.The efficiency of the AFEX pretreatment was quantified by the yield of released sugars during enzymatic hydrolysis of the AFEX-treated wheat straw. The optimal treatment conditions for wheat straw were found to be: ammonia-to-wheat straw ratio, 1:1; temperature, 95°C; moisture content of wheat straw, 70% (dry weight basis); and residence time, 5 minutes. Under these conditions, almost 89% of the theoretical sugars were released by enzymatic hydrolysis of the AFEX-treated wheat straw. The enzymatic hydrolysis results showed the significance of AFEX pretreatment of wheat straw when compared to untreated wheat straw with released sugars yield of only 26 %.