The aim of this announcement is the presentation of the process and results of a study on the morphological and syntactic characteristics of the city plan of Thessaloniki. The study has an euristic and experimental character on the nature of the transformations of the city plan during the twentieth century. Hypothesis 1: The city plan of Thessaloniki, designed by H. Hébrard (after the fire in 1917) is considered as a result of the adaption of a design model upon an application area with a certain inflexibility (i.e. the city before the fire in 1917). Proposition 1: The former (urban) design model is only a methodological supposition. Its main contribution is to give a description of the persued (by Hébrard) morphological and syntactic characteristics of the city plan, with a sufficient clarity. What interests is to be recognized the nature of a transformation process of the former characteristics during the design and application of the final city plan. Hypothesis 2: The general character of the present city plan of Thessaloniki derives its origin from Hébrard's project. Proposition 2: The study of this evolution, under the former process of transformation, has an euristic and an experimental character, according to the direction (past-future) this operation proceeds.