Carbon nanotubes were synthesized by catalytic decomposition of methane using a hydrogen storage alloy of LaNi5 with a CaCu5 type of hexagonal structure as a catalyst. It was found that surface treatment of LaNi5 alloy particle in KOH solution was effective for providing catalytic sites for carbon nanotube growth. The reaction gas of methane could easily penetrate and reach the catalytic sites on metallic nickel through the decomposed loose surface layer of La2O3. The metallic nickel on the surface layer of LaNi5 alloy particle was dominant for growth of carbon nanotubes. The composite of carbon nanotubes with hydrogen storage alloy would be expected to have a potential application in the field of hydrogen energy.