A power conversion system at least includes a filter capacitor, a DC-DC converter, a smoothing capacitor, a housing and a refrigerant flow channel. The refrigerant flow channel is disposed between the housing and a flow channel cover. The refrigerant flow channel includes a converter facing portion at least partially facing the DC-DC converter. The filter capacitor at least partially overlaps with the DC-DC converter when viewed in a first direction from an opposite side of the refrigerant flow channel to the converter facing portion. The smoothing capacitor does not overlap with the DC-DC converter when viewed in the first direction and partially overlaps with the DC-DC converter when viewed in a second direction. At least two fastening sections fasten the flow channel cover to the housing and do not overlap with the smoothing capacitor when viewed in the first direction.