This study focuses on how to maximize fuel economy improvement of I.S.G. while keeping 12V system. With 12V system the maximum gain of fuel economy with I.S.G. is known to be about 3~5% in FTP-75 mode because engine stop is only conducted in standstill idle. But in this study deceleration engine stop has been tried additionally and the optimum condition for deceleration engine stop was found to maximize fuel economy improvement in practical point of view, the result of which is about 8.8% in FTP-75.