Fiber reinforced polymer (FRP) based composite materials are presently enormously used and required in numerous structural and materials based applications. But, when these composites were exposed to different in-service environmental conditions there mechanical, thermal and morphological behaviours were altered. Current exploration is planned to investigate the effect of thermal-shock conditioning behaviour of nano-Al2O3 filled glass fiber reinforced polymeric (GFRP) composites with respect to the mechanical properties. The thermal shock conditioning temperature for the specimens was maintained at + 70 °C for 36 h and further followed by conditioning at - 60 °C for the similar time duration. Tensile tests of the conditioned and unconditioned specimens were carried out at 1 mm/min loading rate. The matrix phase i.e. epoxy polymer was reformed with various nano-Al2O3 weight percentage (0.1, 0.3 and 0.5 wt%) with respect to the epoxy matrix. The 0.1 wt% nano-Al2O3/GFRP enhanced composites revealed higher ultimate tensile strength (UTS) as compared all other set of composites. This improvement in strength value may be attributed to the better distribution of nanofillers in epoxy polymer, which resembles to proper stress transfer from matrix to fiber phase. Dynamic mechanical thermal analyses (DMTA) were carried out in order to identify the viscoelastic properties and glass transition temperature with and without the incorporation of nano-Al2O3 in GFRP composites. Scanning electron microscopy (SEM) of fractured specimens was conducted for the specimens to recognize various failures and strengthening morphologies in the composites. © 2019 Elsevier Ltd. All rights reserved.