Objectives: To demonstrate the relationships between body mass index (BMI) and the clinicopathological features of papillary thyroid microcarcinoma (PTMC) and papillary thyroid carcinoma (PTC). Methods: A total of 810 consecutive patients with PTC (501 patients with PTMC) who underwent total thyroidectomy in 2009-2013 were retrospectively reviewed. Height and weight were used to calculate BMI. Results: Increased BMI was strongly associated with i) extrathyroidal invasion (P < 0.001) and advanced TNM stage (P = 0.005) in patients with PTMC (n = 501), and ii) extrathyroidal invasion (P = 0.001), advanced TNM stage (P = 0.001), and multifocality (P = 0.002) in patients with PTC (n = 810). As compared with normal-weight patients with PTMC, obese patients with PTMC had greater risks of extrathyroidal invasion (OR = 5.214, P = 0.0270), and overweight patients with PTMC had greater risks of extrathyroidal invasion (OR = 2.165, P = 0.0013) and advanced TNM stage (OR = 2.019, P = 0.0137). As compared with normal-weight patients with PTC, obese patients with PTC had greater risks of extrathyroidal invasion (OR = 3.101, P = 0.0172), and overweight patients with PTC had greater risks of extrathyroidal invasion (OR = 1.486, P = 0.0279), advanced TNM stage (OR = 1.650, P = 0.0347), and multifocality (OR = 1.651, P = 0.0054). Conclusions: Increased BMI might elevate the risks of aLtressive clinicopathological features, such as extrathyroidal invasion and advanced TNM stage. Obesity control may play an important role in preventing the development of a ressive PTMCs and all PTCs.