Abstract: Carbon nanotubes（CNTs）were synthesized at different pressure in He/C₂H₂ gas mixture by arc discharge.The morphologies of CNTs were characterized by field emission scanning electron microscope（FE-SEM）,field emission scanning and transmission electron microscope（STEM）,X-ray diffraction（XRD）and Raman spectroscopy.Optical emission spectroscopy（OES）was used in situ to investigate the formation process of CNTs.FE-SEM observation reveals that the length of CNTs synthesized in He/C₂H₂ gas mixture is more than 50 μm and some carbon particles deposit on the wall of CNTs.STEM characterization illustrates that the CNTs wall synthesized at 0.100 MPa is obviously bigger than that synthesized at 0.035 MPa.OES results show that CH and C₂ species might act as the formation precursors of CNTs with H atom acting as etching species of amorphous carbon.The consumption rate of anode and product deposit rate on cathode increase with increasing pressure in the reactor.Thus,it is possible to increase the deposit rate of CNTs at higher pressure in the arc reactor by enhancing the evaporation rate of anode from the anode and C₂H₂ as well as the deposit rate of CH and C₂ species.