NOVELTY - The structure (10) has a frame for supporting a vehicle floor and including a front portion (14F), an inclined portion and a rear portion (14R). A battery unit (20) is placed under the floor along vertical direction of a vehicle. A coupling portion couples the inclined portion or another frame connected to the inclined portion to the battery unit. A brittle portion is provided at the coupling portion, and releases the inclined portion or the latter frame from a connection with the battery unit when a forward load along front-rear direction of the vehicle is applied to the coupling portion.    USE - Battery mounting structure for a vehicle e.g. electric vehicle, gas hybrid vehicle and fuel cell hybrid vehicle.    ADVANTAGE - The inclined portion of the frame is released from the connection with the battery unit when the forward load along the front-rear direction of the vehicle is applied to the brittle portion such that possibility that the battery unit is deformed, following the deformation of the inclined portion of the former frame is reduced. The structure allows a slant wall and a battery-side fixing portion to be connected together by a pair of base reinforcements so as to reduce possibility that a boundary portion between the slant wall and the battery-side fixing portion is lifted, thus increasing connection strength between the battery-side fixing portion and the battery unit in an effective manner.    DETAILED DESCRIPTION - The brittle portion is a groove, slit and a notch. The former frame is a rear side part (14). The latter frame is a center cross part (16). The inclined portion is a kick-up portion (14K). The coupling portion is a battery bracket (40).    DESCRIPTION OF DRAWING(S) - The drawing shows an exploded perspective view of a rear body structure of a vehicle to which a vehicle battery mounting structure is applied.    Battery mounting structure (10)    Rear side part (14)    Front portion (14F)    Kick-up portion (14K)    Rear portion (14R)    Center cross part (16)    Battery unit (20)    Battery bracket (40)