Abstract: By analyzing ISO 9141 diagnostic protocol, we developed a protocol conversion circuit board and PC was developed based on diagnosis applications, toward onboard diagnostic (OBD) system on vehicles which use this protocol. Diagnostic data was collected by communicating with Vehicle Electronic Control Unit, which use the Data Link Connector(DLC). Software components was designed with Visual C++ programming, which makes serial communication module can provide a good application software interface to achieve a better processing of data and analysis of data purposes. Diagnostic software using MFC dialogbased programming, to achieve diagnostic information collection, including fault code reading and eliminating, data flow display and analysis, real time dynamic curves show functions.