The effects of Nb element addition on glass formation and thermal stability of Zr65(Cu0.4Al0.3Ni0.3)35-xNbx(x=0, 3,5,6,7) (at%) alloys were investigated through structural and thermal analysis. A composite microstructure with highly uniformly distributed and well-developed crystalline dendrites was successfully prepared by casting the Zr65(Cu0.4Al0.3Ni0.3)28Nb7 melt into a copper mold. The addition of Nb element results in the significant narrowing of supercooled liquid region. Furthermore, the addition of Nb element leads to an increased disorder in atomic structure of the alloy hence increases the thermal stability of the alloy.