PROBLEM TO BE SOLVED: To obtain a vehicle electric power conversion system capable of blocking or reducing heat of a transmission which is transmitted via a housing.SOLUTION: An on-vehicle electric power conversion system is attached to a transmission housing 5 and includes: an electric power conversion system body part 10 having an inverter function which converts direct current power of a battery into alternating current power; an electric power conversion system housing 9 in which the electric power conversion system body part 10 is stored in a storage part provided at the attachment surface 9a side and a coolant passage 14 is formed on a surface opposite to the storage part, the electric power conversion system housing 9 having an attachment surface 9a attached to a fixing part 5a formed in the transmission housing 5; and a resin lid 16 which is located between the transmission housing 5 and the electric power conversion system body part 10, is fixed to the electric power conversion system housing 9, and covers the electric power conversion system body part 10.
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