The compressive strength of fly ash-based geopolymer mortar (FBGM) made from five different sources of fly ash was examined. The weight ratio of alkaline activator to fly ash content (AL/FA), the weight ratio of sodium silicate to sodium hydroxide (Na2SiO3/NaOH) and concentration of the NaOH were considered the main parameters. A total of 180 FBGM mixes were prepared and tested for compressive strength. The results indicated that the optimum AL/FA ratio for activating fly ashes was in the range of 0.5 and 0.6. The compressive strength of the FBGM that were mixed with different fly ashes exhibited different response to the increase of the Na2SiO3/NaOH ratio and NaOH concentration. Fly ashes with a high content of fine particles and amorphous components such as SiO2 and Al2O3 activated at a low dosage of Na2SiO3/NaOH ratio and NaOH. While the activation of fly ashes with a low content of fine particles and amorphous components required a high dosage of Na2SiO3/NaOH ratio and NaOH. An artificial neural network was developed and the analytical results showed high performance in predicting the compressive strengths of the FBGM.