Synthesis of geopolymer matrixes involved utilization of aluminosilicate materials and alkali activators as the main starting materials where the former could be fly ash, slag or kaolin and the latter commonly used are sodium hydroxide (NaOH) and sodium silicates (Na2SiO3) respectively. In this work, fly ash was chosen as one of the raw material in geopolymer preparation. This is due to the fact that millions tons of fly ash generated yearly by coal-fired power plants and the management of this substance is part of the matter of concern. Hence, one of the approaches to address this issue is by turning the industrial byproduct (fly ash) into a valuable resource material in geopolymer production. This paper will further describe the effect of NaOH concentration towards the properties of fly ash based geopolymer product. Geopolymer samples were prepared under fixed curing temperature (60oC) and curing time (1 day). The influence of NaOH concentration in the range of 4 M to 18 M was systematically studied using Fourier Transform Infrared Spectroscopy (FTIR) for structural elucidation, Scanning Electron Microscope (SEM) to observe the morphology and determination of the mechanical properties (flexural strength) was carried out by Universal Testing Machine (UTM). Based on the results obtained, the optimum NaOH concentration (12 M) at which geopolymer exhibits the best mechanical properties was attained. © 2016 The Authors.