ISG (Idle Stop & Go) system for Automatic-transmission equipped vehicle is developed for fuel saving consumed during idle stop. For this purpose, we made three kinds of system suit for the vehicle’s market positioning. One, called semiautomatic version, for small & TAXI vehicle and it operates when the driver changed the shift lever from D to Neutral. The second version, called automatic version, for conventional vehicle and it operates without any shift lever operation. The third one is Belt-Driven system and it developed only for Luxury class vehicle because of its NVH characteristic and higher cost. For these one, we developed our own Software and test procedure. And as a result, we can get more than 6%~30% of fuel economy improvement and eco-friendly brand image.