The density of the three ultramafic melts, PHN1611, IT8720, and MA at high pressure was determined by the sink-float method using olivine and diamond as density markers. Using these data, we obtain K and K’ of the Birch-Murnaghan equation of state for the compression curves of the silicate melts. The density relation between the mantle minerals and the partial melt provides a possible fractionation mode in the deep mantle; accumulation of olivine and pyroxene could occur in the region just above the transition zone, and separation of the partial melt and garnet could provide a potential mechanism for enrichment of the basaltic component in the transition zone. The density relation between diamond, the mantle minerals, and the melt suggests a possibility for accumulation of diamond in the transition zone. Recent discovery of the high-pressure minerals as inclusions in diamond may be consistent with the present model of accumulation of diamond in the deep mantle. The compositional features of the high-pressure mineral inclusions in diamond are discussed on the basis of the mineral-melt partition coefficients determined in the laboratory. The present analysis implies that the compositions of the high-pressure phases in diamond, especially a high concentration of REE in Ca-perovskite cannot be explained by a simple chemical equilibrium at high temperature above 2000°C, but can be explained by (1) chemical equilibrium at low temperature around 1000-1200°C perhaps in the hydrous slab conditions, or (2) multiple events of partial melting and fractional crystallization, that preclude chemical equilibrium among mineral inclusions.