It is known that zwitterions acquire a positive electrical charge under the isoelectric point (I.P.), which increases its adsorption to hairs. We focused on this unique property of zwitterions to increase the amount of adsorption to hair. The target compound was 2-(dimethyldocosylammonio)ethyl octadecyl ethyl phosphate (DOEP), which shows I.P.= 4.3. The adsorption of DOEP to hair was clearly higher at under its I.P. than at higher pH, and the coefficient of friction decreased at the under I.P..