The Multi wall Carbon Nanotubes (MWNTs) were prepared by catalytic deco mposition of methane at 600 ℃. The raw products of MWNTs were heated for different time in the air at 500 ℃. The qualitative and semi quantitative phase analyses of the samples were performed by XRD measurements. The morphology of the samples were observed by transmission electron microscope and their specific surface areas were measured. After the MWNTs were heated for 4 hours, their walls became clean and thinner, i.e., the inner diameters became larger and the outer ones became smaller. And their dispersing extent was improved. But calcining method could not eliminate the metals and their oxides existing in the samples. The Multiwall Carbon Nanotubes (MWNTs) were prepared by catalytic deco mposition of methane at 600 ° C. The raw products of MWNTs were heated for different time in the air at 500 ° C. The qualitative and semi quantitative phase analyzes of the samples were performed by XRD measurements. The morphology of the samples were observed by transmission electron microscope and their specific surface areas were measured. After the MWNTs were heated for 4 hours, their walls became clean and thinner, ie, the inner diameters grew and the outer first became smaller. And its dispersing extent was improved. But calcining method could not eliminate the metals and their oxides existing in the samples.