This paper present a study on the $lceil$Parere su Architettura$ floor$ of Piranesi published in 1765. In the 18th century, there were fundamental cultural and social changes in Europe. This paper intends to discover the influences of these changes on the field of architectural theory, and it also intends to reveal its characteristics and meanings. The $lceil$Parere su Architettura$ floor$ of Piranesi was composed as a dialogue between two architects, named Didascalo and Protopiro. In this paper, Didascalo represent the Master and Protopiro the Novice(according to the translation of Kaufmann). On the artistic theory of the Enlightenment, Didascalo proclaims as a dogma. Then he inveighs against the rigorists, using strong language quite up the attacks of the Novice. The modernistic Protopiro assails every kind of embellishment. The dialogue non only helps us to understand Piranesi, but also reflects the artistic situation in the 18th century. The summary of the result of this study is as follows: 1. The book of Piranesi represents a controversy of the romanticism against the architectural rationalism. 2. The negation of the originality of the Roman architecture has been considered as the negation of the Baroque architecture that was the artistic base af Piranesi. 3. Piranesi had ideological intention to defend the hegemony of the Italian culture through strengthening of the Etruria-Roman-Baroque cultural tradition.