In this study, the adsorption of a cationic dye, Crystal Violet (CV), from aqueous solution onto Natural Illitic clay (NIC) has been studied in batch system and the equilibrium isotherms were determined. The adsorbent has been characterized by chemical analyses, XRD and FTIR. Four two-parameter isotherm models namely Langmuir, Freundlich, Dubinin–Radushkevich and Temkin, and four three-parameter isotherm models namely Redlich- Peterson, Sips, Khan and Toth were used to analyse the experimental data. In order to determine the best fit isotherm, five nonlinear error functions namely, Nonlinear Chi-square test (X2), Sum of the Squares of the Errors (SSE), Average Relative Error (ARE), Sum of the Absolute Errors (EABS), Hybrid Fractional Error Function (HYBRID), were used to evaluate the data. The error analysis demonstrated that the Sips isotherm model has better described the CV adsorption data onto NIC. Furthermore, the type of adsorption was determined from isotherm parameters, and it was concluded that the physic-sorption is the appropriate mechanism adsorption to CV onto NIC.