Post-treatments on green liquor pretreated biomass have been performed to reduce the enzyme cost in the process of cellulosic bioethanol production. Mechanical refining process has some advantages, e.g. effective size reduction and increase in surface area. Oxygen delignification has been known as an effective carbohydrate-preserving bleaching method, which improves enzymatic hydrolysis. In this study, the PFI refining and oxygen delignification were applied as the post-treatments and the improvement in enzymatic hydrolysis by these post-treatments was investigated. Green liquor pretreatment of mixed hardwood chips was carried out at three different chemical charges and then portions of pretreated biomass were oxygen delignified. Pulp refining by PFI mill was performed at various energy inputs and the low levels of enzyme dosage, 5 and 10 FPU (2.42 and 4.85 mg cellulase/g biomass), were used for the enzymatic hydrolysis. When 10 FPU of cellulase was used, the carbohydrate conversion to monomeric sugars of green liquor pretreated substrate was about 50%. However oxygen delignification and refining improved the conversion rate dramatically to about 78%. High conversion rate was also obtained even at 5 FPU of cellulase when oxygen delignification and refining were both used. Although the refining process improved the enzymatic hydrolysis, the increase in conversion rate was marginal at high depending on PFI revolution, indicating that severe refining condition with high energy consumption is not necessary. The energy consumption of the PFI mill was calculated indirectly based on the correlation between refining energy and pulp freeness to demonstrate the benefit of enzyme dosage reduction.