Abstract: One group neutron perturbation theory is used to analyze the activity distribution of the control rodcobalt absorber after irradiation. Analytical expression of the activity distribution is derived, and calcu-lation is done for the first two cobalt absorbers in HFETR. The measurements of cobalt activity werecarried out using the under water facility. Satisfactory agreement between calculated and measured re-sults shows the effectiveness of calculation method for the activity distribution and the relative worth ofthe control rod cobalt absorber.