In this paper, the adsorption of Orange Bezaktiv (SRL-150), onto Diatomite treated at 100°C (Dt-100) and at 300°C (Dt-300) was investigated in aqueous solution. The crystalline phases and surface study of Dt-100 and Dt-300 was investigated using respectively XRD, SEM, FTIR, and BET. The effect of operational parameters such as pH, contact time and, initial dye concentration was studied. From the obtained results there is not drastic difference in the crystalline phases between the obtained materials Dt-100 and Dt-300. The treatment at 300°C increases slightly the specific surface area from 11.09 to 16.12 m2/g and pores volume. The results of adsorption tests show that the most suitable pH and equilibrium time were respectively 2 and 30 min at an adsorbent dosage of 2 g/L. The kinetic studies revealed that the adsorption results fitted very well with pseudo-second order model (R2 = 0.99 for both adsorbents). Adsorption isotherms results were in accordance with Freundlich.