In this research we studied the adsorption process of Basic Red 46 (BR46) cationic dye onto as-grown (MWCNTs) and modified multi-walled carbon nanotubes (MWCNTs-MOD). MWCNTs were synthesized by the chemical vapor deposition method using ethylene as a carbon source and nanocrystalline iron as catalyst, and oxidized by concentrated nitric acid to give MWCNTs-MOD. The adsorbents were characterized by XRD, TGA, HRTEM, FTIR, BET, and zeta potential measurements. The effects of initial dye concentration (5 to 40 mg L^-1), pH (4.0 to 11.5), and temperature (20, 40, and 60°C) on BR46 adsorption onto MWCNTs and MWCNTs-MOD were studied. The isotherm data were analyzed using Langmuir and Freundlich equations. The equilibrium data fit well the Langmuir isotherm for both MWCNTs and MWCNTs-MOD. The maximum adsorption capacities of BR46 onto MWCNTs and MWCNTs-MOD were 19.5 and 51.8 mg g^-1, respectively. The pseudo first-order and pseudo second-order kinetic models and the intraparticle diffusion model were used to describe the kinetic data. Kinetic studies showed that the kinetic data were well described by the pseudo second-order kinetic model. The experimental results indicated that the maximum BR46 removal could be attained at a solution pH of 11.5 and the adsorption capacity obtained was 23.5 and 57.2 mg g^-1 for MWCNTs and MWCNTs-MOD, respectively. Thermodynamic parameters (ΔG^O, ΔH^O, ΔS^O) were obtained and it was found that the adsorption of BR46 onto MWCNTs and MWCNTs-MOD was an endothermic and spontaneous process.