Structural relaxation around free volume in Zr50Cu 40Al10 bulk metallic glass (BMG) during isothermal annealing at 473, 573 and 673 K which are below glass transition temperature Tg = 675 K have been investigated by positron annihilation lifetime (PAL) and coincidence Doppler broadening (CDB) measurements. The trend of change in positron lifetime, which corresponds to the size of free volume at each annealing temperature, has a good correlation with their density change. These annealing processes obey a stretched exponential relaxation function (KWW: Kohlrausch-Williams-Watts law). The relaxation time t0 and β, used as adjustable parameters in the KWW function, were determined at each temperature. These relaxation parameters depend on the annealing temperature, suggesting the distribution of activation energy for structural relaxation. Moreover, the profile of the electron momentum distribution around free volume derived from CDB spectra during annealing showed no appreciable change at each temperature. These facts suggest that long range chemical ordering, particularly around the free volume, dose not take place essentially.