The permanent method for straightening hair is called Japanese hair straightening or thermal reconditioning. This method was introduced in Japan, and later gained popularity all over the world. In this method, the internal structure of the hair is permanently altered with the use of ammonium thioglycolate.

In this process, hair is first washed with a pretreatment shampoo, which is free of sodium. After rinsing hair thoroughly with lukewarm water, it is blow dried with a blow dryer in medium setting. Hair is then divided into sections. Next, a two-part solution known as a neutralizer is applied to each section of hair. The first part of the neutralizer is applied on dry, detangled hair which enables the release of protein bonds in the hair. This solution is applied only to the hair, not to the scalp. It is left on the hair for around five to seven minutes, depending upon the intensity of curls or waves of your hair. The solution is then thoroughly rinsed off and the hair is blow dried. Once the hair has dried thoroughly, it is divided into equal sections and each section is ironed with a flat iron. This is repeated several times, until the hair becomes poker straight. Then, the second part of the neutralizer is applied and allowed to remain for around ten minutes. It is then rinsed off thoroughly and the hair is blow dried once again. Finally, hair is ironed once more to give it a sleek and straight look.