In this work an attempt has been done on investigating the performance of surface modified gypsum as an adsorbent for removal of methylene blue (MB) from aqueous solution. The Effect of initial concentration of MB, pH, contact time, temperature, and dose of adsorbent on adsorption of MB were assessed. To study the structure and morphology of adsorbents, Fourier transform infrared spectroscopy and Scanner electronic microscopy analyzes were performed. The microstructure study revealed that the gypsum showed a coherent and smooth structure before modifying with potassium hydroxide, but after treatment an adsorbent with porous structure and regular and finer particles was obtained. Equilibrium data were best described by the Langmuir isothermal model. The monolayer adsorption capacity for gypsum and KOH modified gypsum in pH=10, adsorbent dose of 0.1 g, initial dye concentration of 25 ppm, contact time of 30 min and temperature of 25 °C were found as 13.35, 29.24 mg/g, respectively, which is higher than the capacity of adsorption of blank gypsum in the same conditions. The Adsorption kinetic data were well fitted with the pseudo-second order model. Thermodynamic evaluations indicated that the adsorption process was favorable, spontaneous and exothermic.