The transport of radionuclides in compacted bentonite is dominated by the diffusion process. The dry density is an important factor in the diffusion of radionuclides through the compacted bentonite. Through-diffusion tests were performed to investigate the effect of dry density on Sr-90 diffusion. In the diffusion tests the sample used was a bentonite taken from the southeastern area of Korea and the experimental solution was synthetic groundwater spiked with a tracer of Sr-90 (as (SrCl2)-Sr-90). The dry density was adjusted in the range of 1.0-1.7 Mg/m(3). The distribution coefficient of Sr-90 in the compacted bentonite was much lower than that obtained by a batch test. The formation factor and porosity of the compacted bentonite were decreased with increasing dry density. The apparent and effective diffusion coefficients of Sr-90 were in the range of 1.41 x 10(-12) 1.20 x 10-(12) m(2)/s, 1.81 x 10(-11)-1.11 x 10(-11) m(2)/s, respectively, and decreased with increasing dry density of compacted bentonite. It was found for Sr-90 that the higher the dry density increases, the more significant the surface diffusion, as compared with the pore diffusion. The results obtained will be used as basic data for the safety assessment of a repository.