Abstract: Multi walled carbon nanotubes（MWNTs） was purified by concentrated nitric acid with the method of condensate return flow.By electroless plating,Ni was deposited on the surface of MWNTs.The dielectric constant,permeability and R^f curve of MWNTs and MWNTs with Ni coating were measured.The absorbing properties of the microwave absorbing composite materials containing the carbon nanotubes after acid treatment and Ni coating were studied.It is found that the maximum absorbing peak of the microwave absorbing composite material containing the carbon nanotubes with Ni coating is 10.46dB at 6.92GHz and move to lower frequency.The band width（R ＜-10 dB） of the composites containing carbon nanotubes with Ni coating is 2.46 GHz,band width（R ＜-6 dB） is 5.41GHz.Comparing with MWNTs,the maximum absorbing peak of the MWNTs with Ni coating was reduced,but the band width increased.