A balance shaft assembly for a vehicle, which includes a balance shaft carrier disposed under a cylinder block and, includes an oil pan, an upper end of the oil pan being coupled to a bottom of the cylinder block, may include a balance shaft protruding rearward from the balance shaft carrier and rotatably supported by the balance shaft carrier, a balance weight formed on an outer circumferential surface of the balance shaft, and a balance weight receiving portion formed at the oil pan and coupled to the balance shaft carrier to seal the balance weight therebetween.