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In this paper, large single crystals of diamond were synthesized in a pure Co–C system at 5.6 GPa and 1450–1470 °C. The diamond morphology, nitrogen content, and Co-related optical centres were investigated in depth. The results show that the diamonds are mostly bicrystalline or polycrystalline, with different surface defects, and the grown single crystals exhibit an octahedral morphology, with primarily {111} faces. The colour of the diamonds is brownish green-yellow and the nitrogen content is up to 980 ppm. The photoluminescence (PL) spectra show the presence of Co–V centres at 623 nm (V3Co2)0 and 843 nm (V1Co1+) in the diamonds. The intensity of Co-related optical centres at 623 nm and 843 nm gradually decreased with increases in the annealing temperature, and finally disappeared. Interestingly, Co–N complex defects appeared at 545 nm and 581 nm after annealing at 1700 °C, and the 524 nm Co–N complexes only appeared when the nitrogen almost completely aggregated after annealing up to 1800–1900 °C, and their intensity gradually increased with increasing annealing temperature. Furthermore, when nitrogen was removed from the growth system, the Co-related optical centres did not appear in the type IIa diamonds before and after annealing, which indicates that nitrogen impurities play an important role in the formation of Co-related optical centres. [ABSTRACT FROM AUTHOR]