Abstract: A step-by-step sketch recognition method was presented to dissolve the illegibility and uncertainty of pen-based sketch.As the core technique of computer supported sketching,the sketch recognition included three tiers：on-line stroke recognition,sketch beautification from the geometric view, and sketch understanding from the semantic view.A knowledge base system （KBS） with a blackboard performed the recognition task well.So that the rough strokes was transformed into geometric line-elements,tidied up by adding available geometric relations and constraints,and finally turned into regular and precise geometric objects. It has been certified by a prototype sketching system,ISID,with promising effect.