The appearance of the hair is of fundamental importance in today's society. Being in fashion, hair straight and with less volume, consumers that before straighted hair with chemicals products and mechanical strength began to use a heat treatment, in addition to hair dryers: the hot plates ("chapinhas") acting on temperature values around 230°C. This procedure causes not only mechanical and chemical damage but also thermal one, making the hair more fragile. The scope of this study was to evaluate the damage to the hair fiber, in untreated samples and these receiving straighteners/relaxers application of traditional and alternative products.The study was divided into five chapters that evaluated: application of straighteners/relaxers with different active ingredients; mechanical damage, protein loss; thermal analysis and scanning electron microscopy. The hair samples used in all studies were treated as described in the chapter one. Commercial products containing the following active ingredients were used: Sodium Hydroxide, Ammonium Thioglycolate, Guanidine Hydroxide (calcium hydroxide reaction with guanidine carbonate), Formaldehyde and Glyoxylic Acid alone and in combination with Carbocysteine. The use of Formaldehyde andGlyoxylicAcid in straightening/relaxing formulations are prohibited by the National Agency for Sanitary Vigilance. All applied products, straight the hair samples; the procedures that used the hot plates become the hair more straight. The straightening/relaxing based on Glyoxylic Acid and Formaldehyde reduced significantly the hair break point making them more fragile. Most protein loss was observed in the sample treated with Carbocysteine (1.74mg/g hair).In the thermal analysis studies at the dewatering stage, Carbocystein treated samples showed greater weight loss(15.17%), at the protein denaturation stage this treated with Sodium Hydroxide (51.06%) and in the carbonaceous material elimination phase all samples showed mass loss greater than the untreated sample;. The lower peak temperatures were observed in the samples without treatment (630°C) and with Glyoxylic Acid (640°C). Observing the images of electron microscopy is noted the change in the cuticle aspect of the samples showing that the edges were damaged, Guanidine Hydroxide, left further residue: the samples treated with Glyoxylic Acid and Formaldehyde showed the formation of surface film as an "enveloping" fiber. The results suggest that there is not a predominance of a more harmful treatment than other, but those using hot plates(straightening/relaxing acids) emphasize the damage.