The effects of as-deposited iron (Fe) film thickness and the hydrogen (H₂) annealing time on the spin-capability of carbon nanotube (CNT) forest have been studied. Both, the as-deposited Fe film thickness and the H₂ annealing time significantly changed the morphology of Fe nanoparticles (NPs) after annealing process during the synthesis step of spin-capable carbon nanotube (SCNT) forest. The spin capability of CNT forests depended heavily on the different thicknesses of Fe films and the H₂ annealing time. In conclusion, the spin-capability of CNT forest can be achieved by controlling the initial Fe film thickness and/or the H₂ annealing time.