It is known that a niobium-carbon Met-Car cluster ion (Nb8C12+) and its intermediates (Nb4C4+, Nb6C7+, etc.) are selectively formed by the aggregation of the Nb atoms in the presence of hydrocarbons. To elucidate the formation mechanism, we prepared NbnCm+ with every combination of n and m in the gas phase by the laser vaporization technique. The reactivity of NbnCm+ with H-2 was examined under the multiple collision condition, finding that NbnCm+ between Nb2C3+ and Nb8C12+ are not reactive with H-2. On the basis of the H-2 affinity of NbnCm+ experimentally obtained, we propose a dehydrogenation-controlled formation mechanism of niobium-carbon Met-Car cluster ions.