The purpose of this paper is to reconstruct the original floor plan and wall design of Seokbulsa Grotto in Kyungju; commonly known as ‘Seokguram’. The paper presents an array of dimensional studies of the existing Seokguram to examine its architectural form, and infers the original floor plan and wall design of Seokbulsa Grotto. Seokbulsa Grotto is designed as a system of 'coherent modules' and was constructed using the dry stone method, which interlocks large stone modules into a whole that becomes the load-bearing structure itself. The design principles governing Seokbulsa Grotto are the spatial axis of symmetry, modular coordination, and the layout grid of a quarter Tang-Ruler(TR: 唐尺). Dimensional studies were conducted with these governing principles in mind and concludes the following about the original floor plan design. In the main chamber, Ansang-stone’s radius is 12 TR, and Flagstone’s radius is 121/4 TR. In the front chamber, the width between the two Ansang-stones facing each other is 22 TR and the longitudinal space depth is 12 TR, while the width between the two Flagstones facing each other is 221/2 TR and Flagstone’s depth is 12 TR. In the passageway, the width between the two Ansang-stones facing each other is 111/2 TR and longitudinal space depth is 9 TR, while the width between the two Flagstones facing each other is 12 TR and Flagstone’s depth is 73/4 TR. The distance from the center to the entrance line of the main chamber is 101/2 TR. Therefore, the total longitudinal length of the Grotto is 431/2 TR at the level of the Ansang-stones, and 44 TR at the level of the Flagstones.