KEYWORDSPapillary thyroid cancer, serum lipid level, tumor size, lipid metabolism 2 Abstract Background: Papillary thyroid cancer (PTC) is the most common type of thyroid cancer and the incidence of PTC continued to increase over the past decades. Many studies showed that obesity is an independent risk factor for PTC and obese PTC patients tend to have a relative larger tumor size and higher grade of tumor stage. Obesity is closely related with lipid metabolism, while the correlation between serum lipid and PTC remains unclear. Therefore, this study aimed to investigate the association between serum lipid level and PTC.Methods: We retrospectively analyzed 1018 PTC patients diagnosed and treated in our hospital, all these cases were first diagnosed with PTC and had complete clinical information including ultrasound reports before surgery, serum lipid (CHOL, TG, HDL-c, LDL-c, Apo-A1, Apo-B, Apo-E, Lp (a)) results, surgical records and pathological reports.Results: All these serum lipid biomarkers did not associated with tumor size in the female group. In the male group, on crude analysis, Apo-A1 and Lp (a) showed a marginally significant association with tumor size OR and 95% CI 0.158 (0.021-1.777), p=0.072; 0.997 (0.993-1.000) p=0.082 respectively.After adjusting for age and multifocality, Apo-A1 and Lp (a) showed a significant association with