Triangular diagrams showing the trends and fields of the ratios of V , Ni and Co in some igneous rock types, oceanic tholeiites and rock-forming minerals are presented and their significance is discussed. Typical igneous rock types and rock-forming minerals occupy characteristic fields in the diagrams. It is clear that there are different trends for the ratios of V, Ni and Co and V/(V+Ni+Co) ratios to Si contents among various differentiation series. It seems that such diagrams are suitable for showing chemical variation in geochemical process.