Abstract: Based on the Ostwald ripening process, a mechanistic model for the fission gas bubble growth has been proposed in this paper. Taking U-Zr alloy and UO₂ ceramic fuel as samples, the variation of FGB porosity and mean bubble size with the burn-up at different irradiation temperature and external restraint have been investigated in detail. For ceramic UO₂, the porosity increase rate with the burn-up at 700°C is 25% faster than that at 400°C. The external restraint impedes the growth of FGB, but it is not very significant. The porosity and mean bubble size in U-Zr alloy for the same irradiation parameters are much greater than that in the ceramic UO₂ because the plastic deformation takes place very easy and quickly for U-Zr alloy during irradiation.