Here, we employ isothermal annealing on a ZrCuNiAl bulk metallic glass (BMG) to obtain different structural states. The nanohardness of the studied Zr-based BMG shows a clear transition from heterogeneous to homogeneous distribution and the Poisson's ratio gradually decreases with prolonging the isothermal annealing time. Then, the structural change is analyzed using high-resolution transmission electron microscope. The structure-Poisson's ratio relation can be quantitatively illustrated by a rule of mixture. These findings have implications for better understanding the structure-property relation from atomic level and thus exploring high-performance BMGs of excellent strength and ductility.