Straw fibers reinforced geopolymer, a new type of metakaolin-based geopolymer, was synthesized in a specific formulation with rice straw fibers added. The straw-geopolymers were soaked in sulfuric acid solution at pH of 1.0, 3.0 and 5.0, respectively, and sodium hydroxide solution at pH of 9.0, 11.0 and 13.0, respectively. Compressive strengths and microstructural changes were studied. The results showed that after being immersed in acid solution at a pH of 1.0 or alkali solution at a pH of 13.0 for 28 days, the compressive strengths of the specimens decreased from 50.28 MPa to 28.90 MPa and 40.00 MPa, respectively. The best performance was observed when the specimens were soaked in sodium hydroxide solution at a pH of 9.0. Scaning electron microscope (SEM) analyses indicated that the lamellar structure of the straw-geopolymer was consistent with pure geopolymer. Cracks which led to a loss of strength were found in the specimens soaked in acid solution at pH of 1.0 and 3.0.