The nanotubes were produced by dc arc-discharge method through helium and argon gas at a controlled pressure ranging from 10 kPa to 80 kPa. The crude nanotubes were oxidized at 770℃ until about 1% of the weight remained and the purified nanotubes were obtained. The electron spin resonance (ESR) of purified carbon nanotubes prepared under different inert gases with different pressures has been measured. The dependence of ESR line shape, linewidth, g value and spin density of the purified nanotubes on different inert gases and pressure is found and discussed.