with regard to fission gas transportation inside fuel rods, the following three mechanisms are important' (1) a localized and time-dependent fission gas release from uo/sub 2/ fuel to the pellet/clad gap: (2) the consequent gas pressure difference between the gap and the plenum: and (3) the inter-diffusion of initially filled helium and released fission gas such as xenon. among these three mechanisms, the second mechanism would result in one-dimensional flow through the p/c gap in the axial direction, while the third averages the local fission gas concentration difference. an attempt was made to develop a computerized model, linus (linear flow and diffusion under un-steady condition) describing the two mechanisms-items (2) and (3). item (1) is treated as an input. the code was applied to analyze short length experimental fuel rods and long length commercial fuel rods. the calculated time evolution of xe concentration along the fuel column shows that the dilution rate of xe in commercial fuel rods is much slower than in short experimental fuel rods. some other sensitivity studies, such as the effect of pre-pressurization, are also presented