Motor vehicle industry is expected to reduce CO2 emission more and more for protecting the environment. Alternator, which supplies electric energy to battery and electrical loads when it is rotated by engine via belt, is one of key components to improve vehicle fuel efficiency. That’s because actual one is greatly affected by electrical loads, and improving alternator efficiecy is effective to enhance actual one. We have reduced rectification loss from AC to DC with MOSFET instead of rectifier diode because on voltage of MOSFET is much lower than diode drop, which results in improving alternator efficiency. Control circuit is required to drive MOSFET because it is an active element. It is important to turn MOSFET ON and OFF during rectification period “synchronous control”. It is turned ON while a rectifier current flows through MOSFET as alternator output, and turned OFF while the current doesn’t flow to avoid drawing in a reverse current from battery. We turn it off so as to remain certain conduction period through body diode of MOSFET before rectification end. It is controled by making a feedback process to coincide with internal target conduction period based on the rotational speed of alternator. The control circuit and MOSFET were molded in the same package as MOS module, which lead to a high environmental durability. In addition to reduce rectification loss, reducing copper loss with low turn stator and iron loss with magnetic steel contributed to improve alternator efficiency by 10%. As a result, we achieved 2.6g/km CO2 reduction with electrical load.