It is known that ion-assisted reaction method is effective for the surface modification of polymers. The surface treatment of Spectra fibers using the ion-assisted reaction method was investigated in the present study The Spectra fibers were treated by $Ar^{+}$ / ion irradiation under oxygen environment. The treatment was carried out at different $Ar^{+}$ ion doses. The$ Ar^{+ }$ /ion doses used were $6$ imes$10^{15}$</TEX> , $1$ imes$10^{16}$</TEX> , $5$ imes$10^{16}$</TEX> , $1$ imes$10^{17}$</TEX> / ions/$m^2$. Optimal $Ar^{+}$ ion dose in the treatment of Spectra fibers was determined by measuring the tensile strength and modulus of Spectra/vinylester composites as a function of ion dose. It was found that the optimal ion dose was $1$ imes$10^{16}$</TEX>ions/$m^2$. It was also found from the scanning electron microscope examination that the surface-treatment improved adhesion between fibers and vinylester resin.