Geopolymer has been widely used as adsorbent for heavy metals and dyes. Modification on geopolymer surface with cationic surfactant can improve the anion exchange capacity of geopolymer. In this paper, fly ash-based geopolymer had been modified with cetyltrimethylammonium bromide (CTAB) which is cationic surfactant and applied as adsorbent of methyl orange (MO) anionic dye. Modified geopolymer had shown better performance as MO dye adsorbent compared to unmodified geopolymer. The adsorption of MO dye showed the best result at low pH and reached equilibrium after 90 minutes. On the basis of kinetics and isotherm studies, MO dye adsorption by modified geopolymer followed pseudo-second order model and Langmuir model with maximum adsorption capacity of 19.231 mg.g-1.