Abstract A method of evaluating the radionuclide content of low-level drum package waste using emission computed tomography (ECT) and transmission computed tomography (TCT) has been investigated. This method requires accurate correction for the γ-ray attenuation in the drum. The TCT and ECT analyses are performed according to the constrained least-squares technique. Satisfactory results were obtained in preliminary experiments carried out using test equipment with 20 NaI(Tl) scintillation detectors and sample 200-L drums which contained sealed γ-ray point sources of 60Co and 137Cs, and pipes and concrete blocks. The radioactive content of one or two 60Co sources (about 3.5 MBq) in a drum of about 200 kg was evaluated in 6 × 6 × 9 voxel with an uncertainty of about ±15% for an ECT measurement time of about 12.5 min.