The present invention relates to a hair treatment composition comprising a multi-arm polyethylene glycol derivative (A) containing two or more functional groups that may be covalently bound to amine, a hair treatment composition comprising a polyethylene glycol derivative (B), containing one or more functional groups that may react with the functional groups of said composition, and/or a biocompatible polymer (C), a hair treatment agent comprising these compositions and a method for using the same. The hair treatment agent of the present invention is capable of increasing the hair thickness and volume, restoring the damaged hair and lasting the hair shape and curls for a long time, by firstly covalently binding some functional groups of said polyethylene glycol derivative (A) to amino acids present on hair surfaces and then covalently binding some functional groups not covalently bound to hair, to a polyethylene glycol derivative (B) and/or a biocompatible polymer (C).
