Ni Zr Al system exhibits a broad temperature interval between the glass transition and crystallization temperature, giving thus a possibility of studying microstructural changes taking place both in amorphous and in the undercooled-liquid states. The changes in positron annihilation lifetime spectra in thermally treated Ni 2 5 Zr 5 5 Al 2 0 alloy, as observed previously (3), indicating that positrons annihilate at places with different properties, have been supported by resistivity measurements. Sequential annealing above and below T g is found to activate different micromechanisms controlling microstructural atomic rearangements and affecting the properties of the alloy; the influence of such treatment on nucleation mechanism is discussed.