In the present work, bulk Cu50.3Zr49.7-xNbx(x=0,2) amorphous alloys 2　mm in diameter were produced by copper mold suction casting. The effect of Nb addition on the glass transition as well as crystallization kinetics of the binary Cu based amorphous alloy has been investigated by using differential scanning calorimetry (DSC). It is found that the alloy containing 2%Nb has a lower fragility parameter m and higher crystallization activation energy Ec, which suggests that the thermal stability and hence the glass forming ability of the surpercooled metallic liquid for the Cu based alloy are enhanced by the addition of very small amounts of Nb element.