The present invention relates to a composition for dyeing keratin fibres, comprising: a) one or more liquid fatty substances b) one or more amphoteric surfactants of formula (I) below: R a′-C(O)—NH—CH2—(CH2)n—N(B)(B′)(I) in which: B represents the group —CH2—CH2—O—X′ B′ represents the group —(CH2)ZY′< with z=1 or 2 X′ represents the group —CH2—C(O)OH, —CH2—C(O)OZ′, —CH2—CH2—C(O)OH or —CH2—CH2—C(O)OZ′, or a hydrogen atom Y′ represents the group —C(O)OH, —C(O)OZ″, —CH2—CH(OH)—SO3H or the group —CH2-CH(OH)—SO3-Z″ Z′ and Z″ represent, independently of each other, a cationic counterion derived from an alkali metal or alkaline-earth metal, such as sodium, an ammonium ion or an ion derived from an organic amine Ra′ represents a C10-C30 alkyl or alkenyl group derived from an acid Ra′—C(O)OH, which is preferably present in copra oil or in hydrolysed linseed oil, an alkyl group, especially a C17 group and its iso form, or an unsaturated C17 group and n represents an integer ranging from 1 to 10 and preferably from 1 to 5, or quaternized forms thereof c) one or more thickening polymers chosen from polymers bearing sulfonic unit(s), polymers bearing sugar unit(s) and mixture thereof d) one or more oxidation dye precursors and e) one or more chemical oxidizing agents, The present invention also relates to a process using this composition. The present invention relates to a composition for dyeing keratin fibres, comprising: a) one or more liquid fatty substances b) one or more amphoteric surfactants of formula (I) below: Ra′—C(O)—NH—CH2—(CH2)n—N(B)(B′)(I) in which: B represents the group —CH2—CH2—O—X′ B′ represents the group —(CH2)ZY′, with z=1 or 2 X′ represents the group —CH2—C(O)OH, —CH2—C(O)OZ′, —CH2—CH2—C(O)OH or —CH2—CH2—C(O)OZ′, or a hydrogen atom Y′ represents the group —C(O)OH, —C(O)OZ″, —CH2—CH(OH)—SO3H or the group —CH2—CH(OH)—SO3—Z″ Z′ and Z″ represent, independently of each other, a cationic counterion derived from an alkali metal or alkaline-earth metal, such as sod
