This work presents a facile and low-cost approach to prepare nanodiamond@carbon nitride hybrid with loose porous carbon nitride layers close wrapped on well dispersed NDs (ND-CN) through the thermal polymerization of the mixture composed of hexamethylenetetramine (HTM) and ammonium chloride adsorbed on dispersed nanodiamonds (ND), in which hexamethylenetetramine serves as both carbon nitride precursor and dispersing agent and the ammonium chloride acts as gas template. ND-CN shows 1.9 and 1.2 times much higher styrene rate for oxygen and steam-free direct dehydrogenation of ethylbenzene than the pristine ND and the previously reported carbon nitride layers encapsulated ND (H-ND) formed by the thermal polymerization of mixture composed of HTM and ND, respectively, ascribed the more exposed active sites containing structural defects and ketonic carbonyl groups and the not weakened synergistic effect of carbon nitride with ND that originating from the enlarging surface area and pore volume owing to the characteristic concerning loose porous structure of carbon nitride layers.