The purpose of Green Building Certification System is to promote the environment-friendly building construction as the life-cycle environmental performance of a certain building is assessed and certified nationally(domestically). By the way, a building can get the green building certificate just with the minimum standard in prerequisite assessment criteria and with the designated scores in totals. GBCS consists of 9 assessment issues and it has 4 grades. Nevertheless the 1st. grade green building has uneven scores in each issue. In other words, it has even 0 point in a special issue. Due to the above problem, the discrimination and reliability in the GBCS grading and scoring has to be secured. This study aims at a new grading and scoring system equipped with the discrimination and reliability through analysing the scores of prerequisite assessment criteria and the scores by issues in the actually certified green office buildings.