Diamond quantum dots (diamond QDs) have attracted great interest on account of their excellent physical and chemical properties. Here, we synthesized the diamond QDs 2–8 nm in diameter through alkaline assisted oxidation at room temperature. This composite method fairly original, simple, low-cost, and eco-friendly. High-resolution TEM, XRD and XPS confirm the diamond structure of the synthesized nanocrystals. The diamond QDs exhibit concurrent multiband luminescence across the whole visible region. The study based on the photoluminescence, UV–Vis absorption, and Fourier-transform infrared reveals that the fluorescence emission originates from the C–O–C and C(O)O related surface defects.