In this paper, an octree geometric modeler is presented. It builds solid models on the basis of geometrical constraints; in fact, the system contains a set of routines which compute the distance from points, straight_lines and lines and define the part of the space cut by a plane or a surface. Besides, the system allows the combination of constraints in a boolean condition for building more complex solids. This system offers an object-oriented shell which allows the user to take advantages of the declarative and procedural paradigms of object-oriented languages for solid modeling. A solid is represent by a tree of objects (a composite object) where the root object contains the properties characterizing the full solid and the leaf objects contain the description of its elementary parts. The octree model is built from this object-oriented representation through "methods".