This paper evaluates the relationship between the compressive strength and microscopic structure of geopolymer mortar using 100% of fly ash instead of cement. The experimental variable is the curing temperature, which may influence the compressive strength of the geopolymer. The compressive strength, porosity, XRD, SEM and EDS are examined after 48 hours of curing at 30, 60 and 90°C. The resulting compressive strength at 60°C appeared to be the largest. In order to evaluate the voids produced during the polymerization, the porosity was measured and showed also the largest value after curing at 60°C during 48 hours. Furthermore, SEM and EDS analyses verified clear improvement of the microstructure after 48 hours of curing at 60°C. Such result can be explained by the variation of the Si-Al ratio according to the curing conditions, which revealed to be lower for curing at 30 and 90°C than at 60°C and demonstrated that the curing temperature has significantly effect on the compressive strength of the geopolymer.