The solidus, liquidus and volume of residual liquid were achieved by study the solidification behavior of a sub-eutectiod U-Nb alloy. It is found under metallographic observation that their microstructure reveals as the dentritic crystal connected each other and residual liquid separated by solid phase during terminal solidification. Thus, serious micro-porosity form easily in oisolated residual liquid area for the filling channels plugging and residual liquid unable to fill the pore, and the amount of porosity in casting is about 1.6% derived from theoretical calculation. The chemical analysis results showed that segregation coefficients of niobium at intermediate and terminal solidifaction are 0.16 and 0.11 respectively, which indicated that Nb is a strong negative segregation element and it severely cluster in the dendritic area.