Nuclear magnetic resonance (NMR) spectroscopy can probe the atomic-level structure and dynamics of materials, and the characterization of core-shell nanoparticles by solid-state NMR benefits from the information obtained from spin-lattice relaxation time measurements . In addition to nanocrystals, high-field solid-state NMR is suitable for characterizing amorphous, heterogeneous, and disordered nanomaterials . Solid-state NMR probes domain sizes in the sub-nm range that remain challenging to examine with microscopic techniques . As an interesting class of materials, nanoparticles within 1-100 nm size range exhibit physicochemical properties between molecular and bulk scales . Nanoparticles containing a significant fraction of key atoms at the surface can have surface defects and a polydispersity of particle sizes in their sample sizes . The geometric size of diamond nanoparticles can vary from 10 nm to 100 nm, offering opportunities for surface functionalization for versatile applications . Crosspolarization magic angle spinning (CPMAS) is used to selectively observe the 13 C NMR spectra of carbon atoms close to the surface of diamond nanoparticles as the 1 H→ 13 C cross-polarization (CP) process is effective between a hydrogen atom and carbon atom within a spatial separation of 0.7 nm . Cross-polarization efficiencies also depend on the relative mobility of the carbons bonded to and/or spatially close to hydrogens . Conversely, high-fidelity direct polarization magic angle spinning (DPMAS) probes all types of carbons, including the inner core sp 3 carbon atoms in diamond nanoparticles . Importantly, in high-resolution solid-state NMR, magic angle spinning (MAS) averages out anisotropic interactions, exhibiting mainly isotropic interactions . Core-shell architectures provide a stable and fundamental design of nanocrystals and nano-particulates that contain carbons as the primary elemental ingredient, and this kind of NMR experimental analysis offers crucial chemical and spatial information on structural heterogeneity . Furthermore, the spatial distributions of defects and the presence of more electronegative heteroatoms such as N and P in the near-surface layers of nanoparticles can be probed by examining the spin-lattice relaxation rates (1/T1) . This work reports how the 13 C spin relaxation times of commercially available diamond nanoparticles depend on the size of the nanoparticles to explain the spatial arrangements of the core carbon moieties and surface-associated paramagnetic centers of diamond nanoparticles.