The church of San Lorenzo in Turin, among one of the most significant works of Guarino Guarini, has been explored extensively regarding its geometrically complex dome and peculiar spatial experience. The so-called Guarinian devices, such as the injection of Gothic principles to that of Roman, visual illusion, and play of light, recur in this church so as to create a unique experience in a baroque space. However, what remains limitedly investigated, is the principal element that has enabled the visible complexities of the church_ the invisible structure. This thesis as a consequent, attempts to study the structural state, composition, and construction sequence of San Lorenzo as was built originally by Guarini, mentioning the 19th century reinforcements.