This research is carried out an affect of temperature on compressive strength, loss weight and water absorption of fly ash based geopolymer mortar. In mix proportion, the ratio of sand - fly ash in range of 1, 1.5, 2, 2.5 and 3 is considered. The geopolymerization is activated by reaction between alkaline liquid and fly ash under 100°C curing on 8 hours. The result is indicated that geopolymer mortar can be 20 MPa in compressive strength with suitable ratio of sand - fly ash. Hence, geopolymer mortar can be decreased in weight but increased in strength about 50 percent at 2000C for 1 hour. After 1 hour in 600°C, geopolymer mortar can be reduced on weight, strength and raise in water absorption. Moreover, the strength reduced about 30 percent to compare to strength at 200°C.