Abstract: Thickness of radioactive source is an important parameter in the field of gamma quantificational measurement in the strong radiation environment and arms control verification. The passive method of gamma ray measurement for the thickness of radioactive source was researched by the relationship between the intensity ratio of gamma rays from same nuclide with different energy and the thickness of the source. The thickness of radioactive source measured by experiment is in agreement with the actual value, which shows it is feasible that the thickness of radioactive source is measured by passive method.