The determination of Zirconium and Hafnium in presence of each other in geological sample by using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) technique had established, the physical parameters of the instrument were adjusted and the suitable zirconium and hafnium lines were selected to give the most accurate determination, the sensitivity of each line was tested by a solution of 20 ppm zirconium mixed with 1 ppm hafnium to obtain the most sensitive lines which were 339.198 nm for zirconium and 239.336 nm for hafnium. The interference effect was studied which prove the interference effect of the major elements of monazite as Uranium, Thorium, Rare earth elements, Iron and Aluminium on the determination of the elements under studying. The sample used in this work was a mineral of monazite from black beach sand which was upgraded of about 97% purity; the sample was leached by concentrated sulfuric acid (H2SO4) at 250 0C to separate zircon which contains zirconium and hafnium as a residue which dried, weighed and fused by sodium hydroxide for complete decomposition and measured by ICP-OES (Nuclear Materials Authority, Egypt) on the selected lines.