|Abstract||Present investigation is focused to elucidate the effects of CNT modification on the mechanical performance of glass fiber/epoxy (GE) composites at different temperatures and also with various hydrothermal environments. To evaluate the effects of sub- and above- zero temperatures on the interfacial durability of CNT reinforced GE composites, in-situ flexural testing of the samples was carried out at different temperatures. Further, the role of CNT content on the elevated temperature mechanical performance of CNT-GE composites was assessed with different CNT contents (i.e. 0, 0.1, 0.3, 0.5 wt.% w.r.t epoxy matrix). CNT/polymer interfacial gripping at lower temperature allows a higher extent of stress transfer across this interface, thus increases the load bearing capacity. However, 0.1% CNT-GE composite, which exhibited highest strength at room temperature, exhibited the poorest flexural performance at 110 °C. To assess the effect of short- and long- term cryogenic conditioning on CNT embedded glass fiber/epoxy composites, CNT-GE samples of different compositions were dipped in liquid nitrogen bath for different conditioning time durations. Roles of CNT functionalization on the low and elevated temperature mechanical performance of CNT modified glass fiber/epoxy composites has been evaluated by comparing the performance of nanophased composites made from unmodified CNT and carboxyl functionalized CNT. The effect of diffusion temperature on the water uptake kinetics and subsequent degradation behaviour of GE and CNT-GE composites has also been investigated.