Isotope Dilution Mass Spectrometry(IDMS) is one of the analytical method which uses enriched isotope spikes and analyzes the abundance of element by comparison of the spectrum between spiked mass and non-spike mass. Especially, the Thermal Ion Mass Spectrometry with isotope dilution technique (in general ID-TIMS) is the most accurate method of the chemical analysis, which enables us to obtain the data better than 1% in accuracy and precision. In IDMS, enriched isotope spike is one of the most important factor in order to obtain the best data. For rare earth elements, in general, a mixture of ¹³⁸La, ¹⁴²Ce, ¹⁴⁵Nd, ¹⁴⁹Sm, ¹⁵¹Sm, ¹⁵¹Eu, ¹⁵⁷Gd, ¹⁶³Dy, ¹⁶⁷Er, ¹⁷¹Yb, and ¹⁷⁶Lu is used as composite spike. IDMS is very useful in geochronology and REE geochemistry. Especially, it is very effective in studying the “tetrad effect” of rare earth elements in natural samples.