The growth behavior of the Kr bubbles in Kr + irradiated Zr–1.1Nb–1.51Fe–0.26Cu–0.72Ni alloy was studied by in situ transmission electron microscopy. Results show that the distributions of Kr bubbles are not uniform in the zirconium alloy matrix after annealing at 700K, 875K, 1025K, 1125K and 1225K, respectively. The Kr bubbles started to grow slowly when the sample was annealed at a temperature below 1025K. The average sizes of the gas bubbles were observed to grow only about 0.4nm in diameter at 700K for 170min and 0.9nm at 875K for 240min, respectively. However, gas bubbles grew very quickly at 1225K. The largest size of the gas bubbles is over 200nm after 180min annealing. The relationship between the bubble size and the annealing time was obtained at the designed annealing temperature. An empirical formula for calculating the bubble size was suggested by fitting equations derived from the experiment data. Finally, the growth mechanism of the bubbles in the zirconium alloy was discussed.