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 a relationship between its dielectric properties and material characteristics. To this end, dielectric properties of SCD samples synthesized using microwave plasma chemical vapor deposition (MPCVD) systems under different growth conditions are measured. A comprehensive material characterization is carried out using birefringence microscopy, Raman spectroscopy, photoluminescence, and X-ray diffraction to analyze crystal quality, defect distribution, and strain. The experimental results show that the measured tanδ values of the SCD samples all reach 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, this study demonstrates that as the test frequency increases, contributions from defect polarization and interfacial polarization at sector boundaries become more pronounced, resulting in higher overall loss. Interestingly, it is found that some periodic defect structures can partially suppress the phonon-polarization related loss mechanism, thus helping to reduce the tanδ values in some samples. In summary, this work elucidates the multi-faceted origins of dielectric losses in SCD and provides valuable insights and methodological frameworks for guiding the synthesis and processing of diamond crystals with further enhanced dielectric properties, suitable for advanced microwave and terahertz applications.