The structural free-volume of the bulk amorphous Zr41Ti14Cu12.5Ni10Be22.5 alloy and the undercooled liquid range were investigated by X-ray diffraction (XRD) and positron annihilation techniques (PAT). The results show that the positron annihilation lifetime in the as-prepared Zr41Ti14Cu12.5Ni10Be22.5 is about 176 ps, indicating that the structural free-volume with a size larger than that of one lattice vacancy of pure Fe is available in the alloy. The positron annihilation lifetime decreases with increasing annealing time when the alloy is annealed at 400 oC in the undercooled liquid region, which reaches a value of 167 ps with annealing time of 300 min. The structure of the bulk amorphous Zr41Ti14Cu12.5Ni10Be22.5 alloy becomes more compact when annealed in the undercooled liquid region