The extraction equilibrium of samarium (Sm) with diisostearylphosphoric acid (DISPA, HR) was measured at 303 K. The complex, SmR(3), was formed, unlike in the case of dialkylphosphoric acids investigated previously. The extraction rate of Sm with DISPA was measured using a stirred transfer cell. The extraction rates were limited by the diffusion process. The permeation of samarium through a supported liquid membrane containing DISPA as a carrier was carried out based on the above results. Under the experimental conditions, the diffusion of samarium in the film of the aqueous feed or the diffusion of the complex in the membrane phase was anticipated to be the rate-determining step.