On account of the search for the optimal composition and structure-phase state of Zr alloys much attention is paid to upgrade the El 10 (Zr-1 %Nb) and E635 (Zr-1 %Nb-0.35 %Fe-1.2 %Sn) alloys that have proved well in terms of irradiation-induced creep and growth, high strength characteristics, and corrosion. The difference between the alloy properties is determined by their states related to their compositions. The structure-phase state of the Zr-Nb and Zr-Nb-Fe-Sn systems has been studied after heat treatment in the α- and α + β- regions and its influence on the irradiation-induced growth (1IG) during BOR60 irradiation at T =315-350 °C was investigated. A substantial difference has been shown in the deformation effected by IIG of those alloys; it is less for Zr-Nb-Fe-Sn alloys in dissimilar structure-phase states. The incubation period of the accelerated growth stage is determined by the αmatrix composition, the phase state and the initial dislocation structure. Neutron irradiation leads to a redistribution of alloying elements between the matrix and the precipitates, and to changes in the αsolid solution composition. These changes affect accumulation and mobility of irradiation defects, anisotropy and formation of vacancy c-component dislocation loops. The appearance of c-loops usually correlates with an axial direction acceleration of the IIG of tubes conforming to their texture. The basic regularities of the phase transformation have been established: a) β-Nb precipitates in Zr-Nb alloys are altered in composition to reduce the Nb content from 85-90 % to ∼ 50 %, fine precipitates likely enriched in Nb are formed; b) β-Zr precipitates are subject to irradiation-stimulated decomposition; c) Laves phase precipitates change composition (the content of Fe decreases) and crystal structure, HCP to BCC (β-Nb); d) (Zr,Nb)2Fe precipitates having the FCC lattice retain their composition and crystal structure; e) no amorphization of any secondary phase precipitates is observable under the given conditions of irradiation (T = 315-350 °C). Based on the dpa, the results were compared pertaining to Zr-alloy IIG deformation vs. fluence in various reactors at different energies of fast neutrons. The presented graphs enable comparison between the results of numerous experiments and enable predictions of Zr-material behavior in long-term operation and at high burn-up in commercial reactors.