Geopolymer concrete with designed strength of 40 Mpa has been mixed from coarse aggregates, sands and geopolymer pastes. Two kinds of pastes are synthesized from different precursors, i.e. fly ash and dehydroxylated kaolin, using sodium silicate solution as the activator. Compression test pieces of 15x15x15 cm3 of both geopolymer and ordinary Portland cement (OPC) concretes (ASTM C39) have been cast and cured. Curing was done at room temperature for 1 day while Portland cement concretes were immersed in water for 28 days to provide complete hydration. After curing, the samples were immersed in ASTM seawater (ASTM D1141-90) for 7, 28, 56 and 90 days. It is found that geopolymer concretes were in general more durable upon seawater immersion than OPC concrete, This is indicated by the compressive strength retained after immersion. Dehydroxylated kaolin geopolymers show the best performance whose strength did not decrease with time of immersion. The strength of fly ash geopolymers decreased by about 20% during 56-day immersion but did not decrease further. Calcium content is suspected to cause the decrease in strength upon immersion. Kaolin geopolymers containing no calcium showed the best performance, while OPC which consist mostly of calcium silicate hydrates as the strength contributor, showed consistent decrease in strength. It is also found from the experiment that room temperature curing of fly ash geopolymer was slow but continued to progress until 28 days both under dry condition (not immersed) and immersed in water.