A nitrogen plasma treatment protocol has been applied to carbon nanotubes as a way to modulate their magnetic properties by the microwave plasma chemical vapor deposition method. The nitrogen treatment is a simple and effective strategy to fundamentally affect the magnetic performance of CNTs, which is related to the microwave power and the time of the treatment. The best treatment time is 30min with the microwave power of 120W corresponding to the N content (defined N/C atomic ratio in at.%) which is 2.15 at.% and the biggest value of Ms is 0.5227emu/g. Using Raman, XPS and EDS mapping, it is proved that N exists in CNTs and the pyridine N and amine N bonding configurations contribute to ferromagnetic origin. Our study of N-2 plasma treatment doping into CNTs to enhance ferromagnetic properties provides an effective technique for the N doping and massive synthesis of ferromagnetic graphene.