Arrays of high-aspect-ratio diamond nanoneedles display great potential in high-throughput and efficient delivery of drugs and biological molecules to a variety of cells including neurons. In this paper, we introduce a simple technique to fabricate arrays of large-area high-aspect-ratio diamond nanoneedles using an ECR-assisted microwave plasma etching process. To manufacture such structures, a nanodiamond film was deposited on a substrate followed by etching in hydrogen and argon plasma. The feature of the fabricated nanoneedle arrays depends on not only the etching parameters but also the conductivity of the pristine film and substrate. On a silicon substrate, the diamond nanoneedles show a vertical side wall profile with a height of 4-8 μm and a diameter of 70 nm to 1 μm at an optimum bias voltage (-230 V) and pressure (5.8-6.3 mTorr). The aspect ratios can be as high as ~50. On the insulating SiO2/Si wafer substrate, high-aspect-ratio nanoneedles can also be obtained and, attractively, are able to be easily delaminated from the substrate without damaging the nanoneedle structure. To understand the growth mechanism, we studied the etching profiles of the nanostructures over the etching duration. It was found that, when thick nanodiamond films are etched in the plasma on a Mo substrate holder, Mo and β-Mo2C nanoparticles can be self-generated and distribute on the diamond film surface. Such self-masking effects result in the formation of diamond nanoneedle arrays by reactive ion etching without the necessity of any artificial etching mask. This simple and high-throughput fabrication technique for diamond nanoneedle arrays paves the way to their application in intracellular delivery and nano-biotechnology.