Diamond holds significant application potential in microwave and deep-space observation windows due to its exceptionally low dielectric loss. This study aims to systematically investigate the key factors influencing the dielectric loss tangent (tanδ) of single-crystal diamond (SCD) and to establish correlations between its dielectric properties and material characteristics. To this end, dielectric property measurements were performed on SCD samples synthesized using microwave plasma chemical vapor deposition (MPCVD) systems under different growth conditions. A comprehensive material characterization was carried out using birefringence microscopy, Raman spectroscopy, photoluminescence (PL), and X-ray diffraction (XRD) to analyze crystal quality, defect distribution, and strain. The experimental results show that the measured tanδ of the SCD samples reached a minimum value of 4.94 × 10^-5. Detailed analysis reveals that the dielectric loss in SCD is attributed to a combination of factors: the density and distribution of internal defects (e.g., vacancies and impurities), the presence of internal growth sectors and boundaries, and phonon polarization losses induced by lattice vibrations under an external electric field. It is conclusively identified that defect density is the predominant factor governing dielectric loss. Furthermore, the study demonstrates that as the test frequency increases, contributions from defect polarization and interfacial polarization at sector boundaries become more pronounced, leading to higher overall loss. Interestingly, it was found that certain periodic defect structures can partially suppress the phonon-polarization related loss mechanism, thereby contributing to a lower tanδ in some samples. In conclusion, this work elucidates the multi-faceted origins of dielectric loss in SCD and provides valuable insights and a methodological framework for guiding the synthesis and processing of diamond crystals with further enhanced dielectric properties for advanced microwave and terahertz applications.