The effect of CO2 laser treatment before dyeing (LD) and after dyeing (DL) was compared in terms of colour variation. Two reactive dyes, CI Reactive Blue 19 (RB19) and CI Reactive Black 5 (RB5), were used in three concentrations −0.1, 1, and 5% owf. Morphological changes of the fabrics after laser treatment were observed using a scanning electron microscope. Chemical modification of the laser‐treated cotton fabrics was detected using attenuated total reflectance Fourier transform infrared spectroscopy. The difference in colour was evaluated by spectrophotometer; reflectance value, colour yield, CIE L*a*b*, and levelness were determined. The tensile strength of fabrics laser treated before and after dyeing was also measured. The results show that the DL approach has a stronger effect, resembling a fading of colour, yet a higher tensile strength, especially when dye RB5 is used. The LD approach, on the other hand, provides a better colour yield, levelness, and tensile strength when dye RB19 is used.