For the first time, significant improvement of the catalytic performance of nanodiamonds was achieved for the dehydrogenation of ethylbenzene to styrene under oxygen‐lean conditions. We demonstrated that the combination of direct dehydrogenation and oxidative dehydrogenation indeed occurred on the nanodiamond surface throughout the reaction system. It was found that the active sp²–sp³ hybridized nanostructure was well maintained after the long‐term test and the active ketonic carbonyl groups could be generated in situ. A high reactivity with 40 % ethylbenzene conversion and 92 % styrene selectivity was obtained over the nanodiamond catalyst under oxygen‐lean conditions even after a 240 h test, demonstrating the potential of this procedure for application as a promising industrial process for the ethylbenzene dehydrogenation to styrene without steam protection.