Active and stable coprecipitated Ni-Cu-alumina catalysts were employed for direct decomposition of undiluted methane to produce COx-free hydrogen and nanocarbon at 773-1013K in a tubular reactor. Methane conversion increased with the reaction temperature, however, the catalyst stability declined. The weight ratio of formed solid nanocarbon and the catalyst employed attained about 380 and 280g C/g cat at 873 and 973 K, respectively. Simultaneously, 30 and 70 vol.% hydrogen could be produced steadily for approximately 70 and 20h, respectively. The doping of copper enhanced the catalyst stability when employed in a suitable amount. However, it made the catalyst particles to become quasi-liquid state at 973-1013K in methane decomposition. Consequently, the catalyst particles were easily cut into small particles and encapsulated by the growing carbon layers. The tendency became pronounced with the increase of copper content. This effect may play an important role for the deactivation at high temperature in methane decomposition.