It is well known that the coercivity of Nd-Fe-B sintered magnets is easily influenced by varieties of grain boundary phases and the coherency between the Nd₂Fe₁₄B (T₁) grains and grain boundaries. Moreover, the heat treatment after sintering is much effective to improve the coercivity. Therefore, it is important to elucidate the reaction between Nd₂Fe₁₄B grains and those grain boundary phases. In this paper, we tried to analyze the reaction of the Nd₂Fe₁₄B grains and grain boundary phases through a measurement of thermal expansion behavior of Nd-Fe-B sintered magnets, in which the addition of Co, Cu and Al are discussed to the ternary Nd-Fe-B sintered magnets. From our observations, it seems that the thermal expansion behavior shows the solid solubility of additives to Nd₂Fe₁₄B grains and also the melting reaction of grain boundary phases.