Modern techniques such as microwave treatment as clean and uniform treatment in textile processing have been used to make the dyeing process eco-friendly, plus time and cost effective. The present study is concerned with the effect of microwave irradiation on dyeing of cotton fabric using Reactive Blue 21 dye. Irradiated (RC) and un-irradiated cotton fabrics (NRC) were dyed using irradiated (RS) and un-irradiated (NRS) dye solution under various conditions. Optimal radiation parameters were used to explore dyeing pH, temperature, and time for getting darker shades. ISO standard of color fastness were applied onto cotton fabrics dyed under optimal conditions. We observed that microwave treatment of dye solution for 3 min. showed maximum colour strength onto irradiated fabrics at 60ºC for 30 min. in the presence of 2 g/100 ml salt as an exhausting agent. The suggested ISO method revealed that MW treatment improved the fastness to light, washing, and rubbing when different shades were made under optimum conditions. We concluded that microwave treatment has not only improved the dyeing behaviour of cotton but also enhanced the colouring characterization using Reactive Blue 21 dye.