This paper aims to compare and discuss the differences and similarities between the mansion and the diplomatic mission, which was built during 1912 to 1949 in Nanjing. Based on the investigation, 22 mansions and 17 diplomatic missions are selected as study objects. First will be the analysis of the site scale, the plan composition, the entrance and the active line of each site. Next will be a classify of the exterior composition of the mansion and the diplomatic mission through the decision tree. And the last part will be the comparison of the characteristics. Similarity of the scale, plan composition, the entrance and the active line can be found between mansions and diplomatic missions. And differences can be found that, missions have smaller scale with more complexes active line and entrance, and simpler plan composition. Although, diplomatic missions have lager scale, with simpler active line and outstanding entrance. And the exterior space in diplomatic missions surrounds main building with more connections than missions’. Because there were more functions, such as foreign diplomacy, government affairs, residence and sociality, were required from diplomatic missions.