Smoothing method of hair comprising (i) a step of applying to the hair a hair straightening composition containing at least one denaturing agent than 18,1 g / mol molecular mass, and being an urea of formula (I ) wherein: R1, R2, R3, R4 independently represent: a hydrogen atom or an alkyl or lower alkenyl of C1-C4, linear or branched, optionally substituted with a radical selected from: hydroxyl, amino, dimethylamino, carboxyl or carboxamide or N-methylcarboxamide - when R1, R2 and R3 represent a hydrogen atom, R4 may also denote a carboxamide radical methoxy ethoxy 1,2,4-triazolyl cyclopentyl methoxycarbonyl ethoxycarbonyl CO-CH> = CH-COOH phenyl optionally substituted by a chlorine atom or a hydroxyl radical benzyl or 2,5-dioxo-4-imidazolidinyl - when R1 and R3 represent a hydrogen atom, R2 may also represent a hydrogen atom or a methyl or ethyl radical and R 4 an acetyl radical - when R1> R2 => = H, R3 and R4 may also form, with the nitrogen atom carrying them, a piperidine or 3-methylpyrazole or 3,5-dimethylpyrazole or maleimide - R1 and R2 as well as R3 and R4 may also form, with the nitrogen atom leads, an imidazole ring, this denaturing agent being at a molar concentration ranging from 2M and 8M (ii) a step of raising the temperature of the hair, with the help of a heating medium at a temperature between 110 and 250, after the step of applying the composition of straightening and before the step of raising the temperature, allowed to stand for 5 composition to 45 minutes being the denaturant urea of formula (I) the only active hair straightening.Procedimiento de alisado de los cabellos que comprende (i) una etapa de aplicación sobre el cabello de una composición de desrizado que contiene al menos un agente desnaturalizante de masa molecular superior a 18,1 g/mol, y siendo una urea de fórmula (I) en la que: R1, R2, R3, R4 representan, independientemente: un átomo de hidrógeno o un radical alquilo o alquenilo inferior de C1-C4, lineal o ramificado, eventual
