Amorphous ribbons of composition Fe68.5Cu1Nb3Si18.5B9 were produced by melt spun unit. Positron annihilation technique along with DSC and XRD studies has been employed to characterize the nanocrystallization process. XRD results confirmed presence of Fe3Si and Fe2B phases. Two life time components could be fitted to life time spectra of amorphous and heat treated samples. Life time of positron in amorphous matrix was found to be 163.3 ps. Small life time components in nanocrystallized samples could be ascribed to positron annihilation within amorphous and nanocrystalline particles. Larger life time component could be attributed to positron annihilation in interfaces associated with primary and secondary phase particles.