Abstract: User defined feature (UDF), which is a function provided by mainstream CAD systems, can reflect user's specific design intents well and reduce repetitive design activities.Therefore, online exchange of UDFs is an indispensable part of the online integration of heterogeneous CAD systems.An UDF online exchange approach based on neutral modeling command is presented.The neutral modeling command representation of UDFs is presented by synthesizing UDF representation of mainstream CAD systems.Then the translation methods between system UDF and neutral UDF command are proposed and the solution to the translation of UDF difference is emphasized.One test case of UDF exchange among Pro/E, SolidWorks and UG using the proposed approach is given.Experimental results show that the UDF online exchange method can support creation, modification and deletion of UDF among heterogeneous CAD systems effectively.