Abstract: Traditionally the 3D problems of the Block Theory are changed to the corresponding ones in 2D space for solving. Although the amount of calculation can be reduced, the results are not complete. Without considering the effects of different forces, eg. gravity, friction, coagulation, etc., upon the stability of keyblocks, their reliability indexes can not be computed. Combining the Block Theory with the methods of CAGD, the 3D boundaries of keyblocks can be calculated. In this paper, the algorithm and data structure are discussed, which can be used to calculate the 3D boundaries of the largest keyblocks. Such method can be used in various application fields of the Block Theory.