In this research, Color yield 100% cotton fabric dyed with direct dyes was modeled through experimental design Central Composite Design (CCD). The factors chosen were dye concentration, salt concentration, temperature, time and L:R. in order to determine Color yield, K/S and finally F k were calculated. This data were analyzed for modeling 6 diazo direct dyes using NaCl, Na 2 SO 4 and Na 2 CO 3 as electrolytes. Multiple linear regression modeling was based on the significant variables determined by ANOVA. Then Box-Cox transformation was employed to investigate the kind of modification leading to an improvement of the model where recommended, the models were modified and final models were obtained. Finally optimization was carried out of find the condition leading to optimum response (F k ).