Cu50.3Zr49.7-xNbx(x=0, 2) bulk amorphous alloys were prepared by copper mould suction casting. The effect of 2at% Nb addition on isothermal crystallization behavior of the Cu-Zr alloy in supercooled liquid region has been investigated by differential scanning calorimetry (DSC). Results show that the kinetics of isothermal crystallization of Cu based alloys can be well described by the Johnson-Mehl-Avrami (JMA) model. The minor-Nb-containing alloy exhibits longer incubation time for isothermal crystallization and broader reaction exothermic peak as well as lower crystallization rate constant k and Avrami parameter n. Meanwhile, the variation of local Avrami parameter n with crystallized fraction in the Nb-containing alloy is different from that of Nb-free one. This indicates that the Nb addition enhances the thermal stability against crystallization and changes the isothermal crystallization process of the Cu-based alloy.