Due to a lack of fundamental understanding of the surface properties of nanocarbons, tuning their surface active sites for higher selectivity of oxidative dehydrogenation reactions has always been a great challenge for carbon catalysis. In this contribution, annealed nanodiamond was controllably grafted by phosphate, which was demonstrated to be an efficient way to adjust the nanodiamond surface and significantly improve the propene selectivity in the oxidative dehydrogenation reaction. We conducted an in-depth study to explore the role of phosphate modification in the reaction in terms of the interactions between phosphate and carbon surface, the evolution and preferential location of phosphorus species, the promotion mechanism, and the impact on the reaction pathway. The results revealed that phosphate preferentially reacts with the phenol groups initially present on the nanodiamond surface, and then it selectively blocks the defect sites that lead to COx formation with an increased propene selectivity. During this process, the catalyst active sites (ketonic carbonyl groups) were not affected. Such effects originated from the formation of covalent C–O–P bonds on the carbon surface, which was estimated as 15 wt % loading.