<p>Granite spherical blocks are the most common and characteristic block in the mid-slope granitic terrains. The objective of this paper is to examine the difference in the rolling mechanism between the partially buried spherical block and the partially buried rectangular prismatic block in a decomposed granite matrix under extremely heavy rainfall conditions at both theoretical and laboratory scales. The toppling mechanism and the equation for the PB (partially buried in granite residual soil slope) rectangular prismatic block are discussed first. The mechanism and equation consider the rectangular prismatic block in an upright position with only the rear part of the base partially buried. From this analysis, it is evident that the block’s stability against toppling is primarily controlled by two factors: the pore water pressure exerted on it and its own weight. The equation for assessing the rolling stability of a spherical block is derived next through the theoretical analysis. This new analytical formula still indicates that pore water pressure and weight are the key factors determining whether the spherical block rolls. The difference is that, for the spherical block, all of the pore water pressure acting on it contributes to its rolling, whereas for a rectangular block, only the pore water pressures acting on its bottom and rear produce a rolling moment, and the pressures on the two sidewalls cancel each other out. The laboratory results validated the intrinsic difference in the instability mechanisms and the effectiveness of this derived formula in predicting the rolling stability of the spherical block by showing a close correspondence between the predicted and observed rolling stability of the spherical block.</p>