[en] When a gamma-ray spectrometer is used to measure a radioactive sample, the radioactivity distribution in samples may affect the detection efficiency. In order to obtain more accurate content of a radioactive nuclide, the influence of the radioactivity distribution in sample should be estimated. The effect of inhomogeneous radioactivity distribution in I-129 transmutation targets on the detection efficiency was examined employing the MCNP5 code. The ratios of the efficiencies in several extreme situations and various assumed distribution to the efficiency in ideal homogenous distribution were calculated at different sample-detector distances. The calculation results shows that the radioactivity inhomogeneity in samples could not be neglected when the abundance of I-129 was measured by a low energy gamma ray spectrometer, even at a large detection distance, and that the effect of axial inhomogeneity on detection efficiency is much more significant than that of radial inhomogeneity. (authors)