With the physical gas adsorption, the inner diameter of clinochrysotile in nature was studied. The result shows that this technique is more effective than transmission electron microscope (TEM) in the study of inner diameter of clinochrysotile nanotubes. It has the following merits: (1) it can be applied to a lot of samples; (2) the information of size, distribution and variation of the inner diameter of ringent nanotubes can be acquired; (3) according to the N_2 adsorption and desorption processess of ringent nanotubes, the special physical and chemical characteristics can be thoroughly studied; (4)combining with the other methods, we can obtain more information and parameters, such as the outer diameter.