Mineralogy and genesis features of diamonds from placers of NE of Siberian platform has been described in this study. In comparison with others some of studied crystals are unique in terms of carbon isotopic composition and nitrogen content. There is the limiting sector, which reflects the evolution of mantle melt or fluid during differentiation. The data points of rounded diamonds from placers of the northeastern Siberian Platform are plotted far beyond this sector, thus indicating their unusual source. The studied diamonds are enriched in nitrogen (up to 2600 ppm), have a light carbon isotopic composition and contain coesite inclusions. Taking into account these facts, one can assume with a high probability that diamonds from placers of the northeastern Siberian Platform crystallized in subducted crustal rocks. The high degree of nitrogen aggregation based on kinetic models indicates that diamonds remained under upper mantle conditions for a long time after crystallization.