The effect of ammonia etching (AE) pretreatment on the formation of catalyst nanoparticles from homogeneous Ni films deposited by sputtering coater for carbon nanotubes (CNTs) growth is investigated systematically. It is shown that the size and distribution of the nanoparticles depend mainly on the AE pretreatment temperature and time. It is observed that Ni catalyst film is agglomerated by AE pretreatment, resulting in the formation of Ni nanoparticles. We have found a critical temperature and time for AE pretreatment processes to obtain catalyst nanoparticles with the appropriate size and high density. The densely aligned CNTs were successfully grown at 800-850 °C on such nanoparticles by thermal Chemical vapor deposition (CVD).