Abstract: Zirconium (Zr) alloys, owing to their favorable radiation resistance and mechanical properties, are commonly used in materials such as pressure tubes and cladding for nuclear reactors. To investigate the microstructural differences between grains with different morphologies in Zr-2.5Nb alloy subjected to helium ion irradiation at room temperature and 350 ℃, microstructural analysis via transmission electron microscopy (TEM) was conducted on TEM thin film samples of Zr-2.5Nb irradiated at room temperature and 350 ℃ with 50 keV and a fluence of 3×10¹⁶, 5×10¹⁶, and 1×10¹⁷ cm⁻². The evolutionary differences of helium bubbles and dislocation loops between equiaxed (E-type) grains and lath-shaped (L-type) grains under identical irradiation environments were comprehensively explored. Experimental findings reveal that helium bubbles in L-type grains exhibit consistently larger sizes than those in E-type grains under irradiation at 350 ℃. At the irradiation damage levels of approximately 0.3, 0.5 and 1.0 dpa, the size differences of helium bubbles between the two grain types reach 0.12, 0.15 and 0.29 nm in sequence. Nevertheless, no remarkable distinction exists in helium bubble sizes among grains with different morphologies when irradiation is conducted at room temperature. When the irradiation dose rises from 0.3 dpa to 1.0 dpa at 350 ℃, the growth amplitude of helium bubble size in L-type grains is 2.32 nm, while that in E-type grains is 2.15 nm. Meanwhile, the size increment of dislocation loops in L-type grains is 1.57 nm, and the corresponding value in E-type grains is 3.21 nm. At 1.0 dpa, as the temperature increases from room temperature to 350 ℃, the helium bubble size increases by 3.51 nm in L-type grains and 3.28 nm in E-type grains. In terms of dislocation loops, their sizes rise by 1.36 nm in L-type grains and 3.26 nm in E-type grains under the same temperature variation range. The number density of helium bubbles declines with increasing irradiation temperature, whereas the density of dislocation loops presents an opposite rising tendency. At 350 ℃, dislocation loops in L-type grains own larger average sizes than those in E-type grains at 0.3 dpa and 0.5 dpa. By contrast, E-type grains contain larger dislocation loops at 1.0 dpa with a size gap of 1.57 nm. Additionally, the density variation of dislocation loops shows an initial increase followed by a decrease as the irradiation dose rises. The systematic results provide reliable experimental evidence for further understanding the fundamental irradiation damage mechanisms of zirconium alloys, and also offer valuable theoretical references and practical guidance for the optimal design and microstructural modification of advanced nuclear-grade zirconium alloy materials.