Short-lived fission gas release from fuel pellets during irradiation was investigated based on the experimental results of the gas-flow rigs irradiated in the Halden Heavy Water Reactor (HBWR).The release-to-birth (R/B) rates of short-lived fission gas were measured by means of gas-flow measurement during the irradiation experiments. Surface-to-volume (S/V) ratios of fuel pellets and diffusion coefficients of short-lived fission gas release were evaluated from the obtained (R/B) values. The increase of (S/V) ratio agreed well with the point where the fuel temperature exceeded the threshold of 1% fission gas release. This indicates that the interlinkage of fission gas bubbles occurred there. The evaluated diffusion coefficients scattered in the range between 10 −23 and 10 −17 m 2 /s, and the effects of fuel type (UO 2 or MOX) were not clearly observed. In addition, it is likely that the restructuring effect of fuel pellet on the diffusion coefficients of short-lived fission gas at least in the fuel pellet matrix is negligible in high burnup region where the rim structure forms in the fuel pellet.