Comparison analysis of linear least-square method and non-linear method of three widely used isotherms, Langmuir, Freundlich and Redlich-Peterson were examined to experiment Remazol Red (C.I. 18221) onto pistachionut shell at a solution temperature of 293 K. All the three isotherm equations showed a better fit to the experimental equilibrium data. The results show that the Langmuir model shows the best fit compared to Freundlich and Redlich-Peterson models using linear method. However, when using non-linear method, The Freundlich and Langmuir models show the best fit compared to other. Redlich-Peterson isotherm is a special case of Langmuir isotherm when the Redlich-Peterson isotherm constant g was unity.