The purpose of this study is to reveal the evolution process of spatial formation between the Piazza di Spagna in Rome and its circumstances. In order to facilitate the research, 4 situations of different periods and urban contexts around the Piazza di Spagna were reconstructed and compared, in base of various documentations including iconography, cartography, bibliography, photography. In conclusion, the spatial transformation of the Piazza di Spagna is the result of complex situations but the open grand stair called Spanish Steps, designed by the architect Francesco de Sanctis in collaboration with Alessandro Specchi in early 18th Century was the most decisive factor for defining character and function of the piazza today. The piazza itself was not completed at once but formulated over centuries and in urban scale, has the meaning of important node point related to the Strada Felice of the Pope Sixtus V, at the same time to the river Tevere through the Via Condotti. Its spatial form, physically divided in two different triangles, is the key to contain two different characters, that of commercial and that of religious, in the same square.