A vehicle body structure includes a battery mounting part provided below a floor for mounting at least one battery, first and second cross members to reinforce the floor in front of and behind the battery mounting part, respectively, left and right center side members extending in a front-rear direction and connecting the first cross member and the second cross member, wherein the left and right center side members are spaced apart from each other in a left-right direction and reinforce a portion of the floor, a fuel tank mounting part provided below the floor behind the second cross member, left and right reinforcing frames extending rearward from rear ends of the left and right center side members, respectively, to reinforce the floor, and left and right extension members extending rearward from rear ends of the left and right reinforcing frames, respectively, to reinforce the floor.