The effects of different activator concentration, liquid/fly ash ratio, and curing temperature and time on the compressive strength of specimens prepared from low-calcium fly ash activated with sodium hydroxide without the use of Portland cement were investigated. SEM, XRD and mercury intrusion porosimetry (MIP) were used to observed the structural feature, reaction products, and porosity and pore-size distribution of the specimens from alkaliactivated fly ash, respectively. It was found that the degree of reactivity, as shown by the compressive strength, the activator concentration and the ratio of liquid/fly ash, and the curing temperature always result to be significative factors. The 7, 14, and 28-day compressive strengths of specimens prepared from alkali-activated fly ash by 5M NaOH solution at 50 °C are 152, 219, and 263 kgf/cm2, while those from 6M solution are 184, 225, and 267 kgf/cm2, respectively. In SEM observation, the fly ash activated by the 5M NaOH solution shows a more continuous matrix with solid and non porous due to subsequent gel restructuring by amorphous alkaline aluminosilicate produced from alkali-activated fly ash.