PURPOSE: To obtain fibers having high water vapor absorption, water absorption and antistatic properties, by impregnating polyamide fibers with a grafting solution containing a hydrophilic vinyl monomer adjusted to an acidic side, treating and subjecting the monomer to graft polymerization uniformly in high reaction efficiency.

CONSTITUTION: Polyamide fibers are impregnated with a grafting solution prepared by adjusting a grafting solution containing a hydrophilic vinyl monomer [e.g. (meth)acrylic acid] to pH 1≤ to pH ≤4 with an organic acid, dehydrated and the monomer is subjected to graft polymerization. By adjusting to the grafting solution to the acidic side and subjecting to graft polymerization, the hydrophilic monomer can be uniformly subjected to graft polymerization in high reaction efficiency to give modified polyamide fibers having high water vapor absorption, water absorption and excellent antistatic properties. The polyamide fibers are effective as an inner wear having a cooling feeling.