We investigated the treatment of a mixture of three reactive textile dyes (C. I. Reactive Red 195, C. I. Reactive Yellow 84 and C. I. Reactive Blue 69) by electro-Fenton (EF) process using carbon paper cathode modified with carbon nanotubes (CNTs). To study the degradation of mixture of three dyes, a rapid analytical methodology based on recording UV-Vis spectra during the EF process and the data treatment using partial least square (PLS) as a multivariate calibration method was developed. The three reactive dyes were quantified simultaneously despite the overlap of their spectra. Central composite design was used in the modeling and optimization of the electrochemical decolorization process. The independent variables for modeling were initial concentration of three dyes, applied current in the electrolysis system, and reaction time. Results indicated that the optimum applied current value was about 400mA for the EF process. Analysis of variance (ANOVA) showed that predicted responses by CCD match the experimental values logically with the amount of R2 more than 0.92.