The TC of BaμSr1-μTiO3(BST) as a f unction of the composition and the lattice parameter γ is examined. It is compa red with the lattice parameter dependence of TC of BaTiO3 and SrTiO3 respectively. It is argued that the variation of TC in BST can be attributed to the change of cell volume. The order and the di ffuseness of the phase transition are also discussed from the volume effect view point. The fact that the Ba and Sr are ionized leads to the volume effect of BST .