SignificanceUncovering the structural complexity of nanodiamonds has been a significant experimental and theoretical challenge. We show that cubic nanodiamonds of small sizes (e.g., <5 nm) display the characteristic (200) forbidden reflections in their electron diffraction patterns, which makes them indistinguishable from new diamond (n-diamond). Besides the size effect, our study demonstrates that various defects, including distortions, dislocations, and boundaries, can also make the (200) forbidden reflections visible. Thus, we supplement the explanation of n-diamond structure and reveal that using electron diffraction alone is insufficient to differentiate various diamond nanostructures. These findings shed light on the impact of order/disorder on nanodiamond structures at the single-atom level.