Abstract: Carbon nanowalls were modified on the surface of high temperature and high pressure single crystal diamond by electron cyclotron resonance(ECR)microwave plasma under CH₄/H₂system in this paper.The plasma emission spectroscopy was used to study the variation of the spectral intensity of the CH₄/H₂ ECR plasma in different working pressures and CH₄ concentrations.The micro structure of the single crystal diamond was analyzed by scanning electron microscopy(SEM)and the effects of pressure and CH₄ concentration on the modification results of carbon nanowalls were further studied.The results show that the orientation of carbon nanowalls is greatly affected by the working pressure.The carbon nanowalls grown under low pressure(0.07 Pa)have vertical orientation, and the surface of the diamond also has vertical etched morphology.At high pressure(5 Pa), the underlying carbon nanowalls have poor orientation.At the same time, the critical CH₄ concentration of carbon nanowalls growth is also related to the working pressure.The critical CH₄ concentration of carbon nanowalls growing under low pressure is higher.When the working pressure is 0.07 Pa, the critical CH₄ concentration of carbon nanowalls growth is 3%.When the working pressure is increased to 5 Pa, the critical CH₄ concentration of carbon nanowalls growth is reduced to 1%.And the density of carbon nanowalls increases proportionally with CH₄ concentration.