We have found that lanthanide tetrad effect (PEPPARD et al., 1969) is operative for heavy rare-earth elements (REE) in a phosphorite sample (GOLDBERG et al., 1963). Also it has been evinced that the similar effect is observed for REE in sea waters. However, in the case where the plot (Masuda-Coryell plot) is void of the points for Tb, Ho and Tm, the resultant joint-line pattern is to have the common characteristics that the zigzag pattern is pointed downward at Dy and Yb and upward at Er. That is, the zigzag pattern with phase coincidence in breaking and its direction is a partially cut-off reflection of lanthanide tetrad effect. Employing the stable isotope dilution technique, we could determine REE in one-liter sample of sea water, with errors less than 2∼3%.