Modeling free-form shapes in a 3D space is one of the difficult problems in computer graphics. In a parametric patch, intuitive control of free-form shapes is not easy. Modeling methods based on strict physical laws need considerable computation time to deform objects, which are not suitable for interactive applications. In this paper we focus on a cellular automaton and try to apply it to interactive free-form modeling within a 3D voxel space. Each voxel is allocated a finite state automaton which repeats state transitions according to the conditions of its neighbor voxels. Virtual clay objects defined in such an active voxel space are easily deformed under the constraints of physical conservation laws.