This study is to analyze the geometrical composition system for the plan of S. Pietro by Michelangelo. In the result of study, the plan is based on the geometrical elements of circle square cross, and shaped to symbolize the Universe which consists of the Heaven the Earth the Man. The plan is constituted of the conception of composition into hierarchy and repetition intersecting 45 degree the same sized square with inscribed square in a circle. Such the geometrical composition system can be found out a large number of example to the geometrical composition of architecture and city planning from Vitruvius to Bramante and Da Vinci. This plan is disposed in balance as the regularly proportional system of 1:1, 1:2, 1:3, which is formed the principal space. And the interior space is constituted of the organic space system, expanding to the direction of horizontal, vertical(ascension), development for primary space and secondary space with center in the geometric composition system of altar.