〔Abstract〕 Objective To assess and to compare the diagnostic performance of cervical ultrasound and 18F-fluorodeoxyglucose(18F-FDG PET/CT) in identifying cervical lymph node metastases in patients with differentiated thyroid cancer(DTC) following total thyroidectomy, focusing on both early and delayed imaging phases. Methods A retrospective review was performed on 83 DTC patients with DTC who had undergone total thyroidectomy. A total of 143 cervical lymph nodes, comprising both metastatic and non-metastatic nodes, were evaluated using 18F-FDG PET/CT and ultrasound. Sensitivity, specificity, and overall diagnostic accuracy were calculated for each imaging modality. Differences in lymph node detection rates across cervical regions using ultrasound were analyzed, along with nodal size parameters(long and short axis diameters), to investigate potential limitations of ultrasound in this context. Additionally, PET/CT parameters including axial ratio, early and delayed maximum standardized uptake values(SUVmax), and changes in SUVmax between imaging timepoints(ΔSUVmax) were compared between metastatic and non-metastatic lymph nodes to evaluate the diagnostic value and underlying mechanisms of PET/CT in this clinical setting. Results 18F-FDG PET/CT demonstrated a sensitivity of 91.58%(98/107), a specificity of 72.22%(26/36), and an overall diagnostic accuracy of 86.71%(124/143) in identifying cervical lymph node metastases post-total thyroidectomy in DTC patients. In comparison, ultrasound showed a lower sensitivity of 50.47%(54/107), but a higher specificity of 97.22%(35/36), with an overall accuracy of 62.24%(89/143). Notably, ultrasound exhibited a significantly higher false-negative rate in the first cervical lymph node group compared to the second group [56.6%(30/53) vs 43.4%(23/53); χ2= 20.396, Pmax compared to early-phase imaging(Z=-8.147, Pmax threshold of 2.5, delayed imaging identified a greater number of positive lymph nodes than early imaging(χ2=18.127, Pmax values were significantly higher in malignant nodes compared to benign ones(Z=-3.689, Pmax was markedly elevated in metastatic lymph nodes(Z=-4.189, P18F-FDG PET/CT proves to be superior to ultrasound in diagnosing cervical lymph node metastasis in patients with differentiated thyroid cancer(DTC) following total thyroidectomy. Delayed-phase imaging offers higher tumor-to-background ratio, which enhances the visibility of metastatic lymph nodes. In addition, dual-phase 18F-FDG PET/CT plays greater advantages in the differential diagnosis between metastatic lymph nodes and non-metastatic lymph nodes.