A resin for extraction chromatography was synthesized by suspension copolymerization, and its performance in separating four rare earth (RE) elements, gadolinium (Gd), terbium (Tb), dysprosium (Dy), and holmium (Ho) was investigated. The resin was synthesized by changing the reaction rate using a reaction initiator (benzoyl peroxide, BPO). The effects of the concentration of the eluent, HCl, were examined. The separations were best (=1.000, =1.091, and =1.495) when BPO was added in 3 aliquots, and 0.3mol/L HCl was used as the eluent. The synthesized extraction resin gave similar separation resolutions to the commercial resin at lower HCl concentrations than the 0.6mol/L used with the commercial resin. The scanning electron microscopy (SEM) analyses showed that the best-prepared synthesized extraction resin had uniform, spherical particles. The Fourier transform infrared (FT-IR) spectra of the synthesized extraction resins showed the characteristic signals of styrene-divinyl benzene copolymer.