This paper deals with 3D tolerance analysis involving dimensional and geometric tolerances. The tolerances connected with each component of an assembly are simulated by varying randomly the geometry of its part geometric model features. In particular, an original software has been developed to analyse the influence of the tolerances on the geometry of the part by using tools strongly connected with solid modelling. In this way the tolerances are mutually integrated with design reasoning. The approach has been applied to a case study of an assembly with five parts. The effect of the most critical tolerances has been investigated.