The rear floor part structure for a vehicle includes a rear support bracket disposed between a spare-tire compartment and a rear floor cross member, a cross member bracket mounted at a coupling part of the rear floor cross member and rear floor side member by welding, and a pair of side member brackets disposed to face each other and inserted by being welded inside the rear floor side member near the part where the rear floor cross member couples with the rear floor side member is disclosed. The structure constructed above is configured to reinforce the rigidity of the vehicle body, thereby limiting the amount of thrust placed on the rear floor part toward the front of the vehicle when a rear collision occurs.