The germination activity and the initial water absorption characteristics of Welsh onion seeds were examined at different levels of deterioration. Using data on the changes in moisture content of Welsh onion seeds, a non-linear least squares method was applied to an approximate solution of the diffusion equation. The constant for a given solid shape values (B1) of the diffusion model were estimated to be about 0.8. The data were fitted to the exact solution for the infinite plane sheet diffusion model using a non-linear least squares method. The measured results agreed with the calculated results. The specific volume changes of the sample were determined using particle density measurements at several moisture contents by a liquid-displacement method using a pycnometer. Using an empirical equation with three parameters that related moisture content and temperature to specific volume, the measured data agreed with the values calculated from the equation. The results of the comparison of aging effects among the three deterioration levels of Welsh onion seed showed that the water absorption rate of aged seeds was lower than the fresh seeds at all tested temperatures. Moreover, the aging effect appeared as lower volume expansions by water absorption, besides the increase of abnormal and non-germination seeds in the germination test.