Abstract: The irregular voxels of waste drum at the periphery are regarded as regular voxels in tomographic gamma scanning. In order to explore the influence of treatment modes of voxels at the periphery of waste drum on the quality of transmission reconstruction, MCNP code was used to compare the irregular voxels mode and regular voxels mode, with algebraic reconstruction technique (ART) and maximum likelihood expectation maximization (MLEM) respectively. The results show that irregular voxels treatment mode of waste drum at the periphery is preferable, for the reason that by regular voxels treatment mode, there will be a relatively big discrepancy between reconstructed linear attenuation coefficients (LACs) and the real ones, which may cause failure in accurately reconstructing activity in upcoming emission reconstruction stage, and that compared with ART algorithm, MLEM algorithm is a better choice to the transmission reconstruction algorithm.