Abstract: Based on singularity of constraint equation and convex hull division of structure parameter space, the dimension classified method of planar mechanism is researched. The dimension classified inequation is derived from singularity of constraint equation. The characteristic of classified inequation is discussed. Convex hull expression and division method of structure parameter space are researched. The relation between characteristics of convex hull and mechanism type is discussed. The division process of convex hull is discussed. By several examples, the classified method is validated. Research results demonstrate that this method has distinct advantages. On the one hand, all information about mechanism classification is included in all results, the transformation relation of dimension type can be obtained from the results; On the other hand, the Carathéodory’s theorem of convex hull is propitious to confirming the dimension of mechanism. Furthermore this method is very suitable for using computer to process complicated mechanism classification

Key words: Convex hull, Dimension classification, Planar mechanism, Singularity