The chemicals of NaOH is highly related to the quality of the pulp in the medium consistency oxygen process. This research used Acacia mangium and Eucalyptus pulp with KaNo 18. The condition of the medium consistency oxygen process, the temperature of 80^oC, oxygen pressure of 1 bar. Reaction time was 20 to 100 minutes at intervals of 20 minutes. NaOH charges 5 to 25 kg /t at intervals of 5 kg/t. The analysis parameters used are KaNo and viscosity. The higher value of NaOH charge and reaction time, the higher value of the delignification degree. The higher value of NaOH charge and the reaction time, the higher the value of chain scission number. The variation of NaOH charge in the same reaction time indicated that the higher NaOH charge value, the lower value of the polymerization degree. The variation in reaction time indicated the longer the reaction time, the lower the polymerization degree. The reaction selectivity value was not affected by the high value of NaOH charge. The longer the reaction time used, the lower the reaction selectivity.