Zirconium alloy components subjected to long-term operation and high doses in thermal reactors need to be highly irradiation resistant to provide integrity of components, primarily, their geometrical sizes. Transmission and scanning electron microscopy, energy dispersive X-ray microanalysis used to investigate thin foils and extraction replicas of irradiated zirconium, Zr-1Nb (E110) and Zr-1.2Sn-1Nb-0.4Fe (E635) alloys allowed us to analyze the evolution of their microstructure under neutron irradiation. The experimental irradiations that were conducted at 350°C to 10²⁷ n/m² (E ≥ 0.1 MeV) show that the most irradiation resistant alloy proved to be a multicomponent E635 alloy. It is not essentially subject to growth. Dislocation structure and phase composition were studied as interrelated to different stages of irradiation induced growth. The accelerated growth correlates with a high density of basal — plane 〈c〉-component dislocations.