Purification of La and Ce by a horizontal plasma arc-zone melting (PZM) with Ar plasma arc heating under atmospheric pressure was examined. A stable molten zone of La and Ce on the water-cooled copper hearth could be maintained and, after zone passes, the segregation of impurities along the bars was recognized. Metallic impurities such as Fe, Cu, Al, and Si moved in the zone direction and decreased efficiently at the head side of the zone-melted bars. Especially, remarkable segregations of those impurities were observed on La. In addition, volatile impurities such as Ca, Mg, Zn, and Mn were reduced to further low levels by vaporization. For non-metallic impurities, O, N, C, and S in Ce, and O and N in La moved in the opposite direction of zoning, while C and S in La moved in the zone direction. Furthermore, nonmetallic impurities were reduced unexpectedly along the whole bar during PZM. Consequently, the purity around the head side of La and Ce bars could be sufficiently increased with PZM.