PURPOSE: To impart a fiber with high flame-retardance having high durability by impregnating a solution containing a specific compound incorporated with a crosslinking assistant into a fiber, drying the solution and irradiating with electron ray in an inert atmosphere to effect the graft-polymerization of the compound.

CONSTITUTION: An aqueous solution containing a vinyl phosphonate oligomer (a flame-retardant) is incorporated with phenoxyethyl acrylate or ethylene glycol diacrylate as a crosslinking agent. The obtained treating liquid is applied to a fiber or a woven cloth, dried and irradiated with electron ray in nitrogen gas atmosphere to effect the graft-polymerization of the vinyl phosphonate to the fiber and obtain a fiber having high durability and flame-retardance without generating free formaldehyde.