Defects in electrodeposited nanocrystalline Ni and the effect of hydrogen on the positron lifetime have been studied by the positron annihilation technique. There exist three kinds of defects at the interface

i. e. the free volumes with a monovacancy

a cluster of 6-10 vacancies and a micro-void

respectively. It is found that the moderate and longest lifetimes will not be affected when the nano-Ni is annealed at 500℃ in vacuum for 2h

which suggests that the defects related to these lifetimes show surprisingly great thermal stability.