Nanodiamond combines the properties of both nanomaterials and diamond materials, resulting in different characteristics from that of micron and bulk diamond. In this work, the structure, optical properties and thermal stability of diamond samples with different sizes were studied by scanning electron microscopy, X-ray diffraction, spectroscopy techniques and thermogravimetric analysis. The results show that the sizes of three samples are 300 μm, 30 μm and 100 nm, respectively. Diamond samples with larger size present better crystal quality, and are confirmed to be Ⅰb type diamond because single nitrogen impurities are observed. In contrast, the crystallinity of nanodiamond samples is poor, and residual graphite is detected in nanodiamond samples from both XRD and Raman results. In addition, H2O, N—H and C—H bonds are observed in nanodiamond, suggesting there may be many organic reactive groups on surface. Two emission peaks are observed in both samples of 300 μm and 30 μm, originated from two types of nitrogen vacancy defects with neutral and negative charge respectively. In contrast, no emission is observed for nanodiamond because of fluorescence quenching from nonradiative center originated from organic groups and surface defects in the nanodiamond. Diamond samples with large size have the intensive absorption in the range from 300 nm to 525 nm, while the nanodiamond sample show strong absorption in the entire UV-visible-NIR region resulting in much lower transmittance. With the decrease of particle size, both the initial oxidizing temperature and oxidizing rate gradually decline. As a result, larger size samples have better thermal stability.