Objective: To investigate the associations between hematologic parameters and obesity in children and adolescents.Methods: A total of 7,997 subjects (4,259 boys, 3,738 girls) aged 10–18 years was enrolled and hematologic parameters, including WBC, RBC, Hb, Hct, and platelet levels, were recorded and compared against body mass index (BMI) classified into normal-weight, overweight, and obesity groups.Results : The obesity group had significantly higher mean levels of WBC (7.16 vs. 6.16 (x103/mm3), p<0.001), RBC (4.90 vs. 4.82 (x106/mm3), p<0.001), Hb (14.07 vs. 13.99 (g/dL), p<0.05), Hct (42.31 vs. 41.91 (%), p<0.001) and platelets (311.87 vs. 282.66 (x103/mm3), p<0.001) than the normal-weight group after adjusting for obesity and sex. BMI SDS was significantly positively associated with WBC (β=0.275, p<0.001), RBC (β=0.028, p<0.001), Hb (β=0.034, p<0.001), Hct (β=0.152, p<0.001), and platelets (β=8.372, p<0.001) after adjusting for age, sex, and possible socioeconomic confounders in a multiple linear regression analysis.Conclusion: Higher BMI is associated with elevated WBC, RBC, Hb, Hct and platelet counts in children and adolescents. Because higher hematologic parameters are potential risk factors for obesity-related morbidity, more attention should be paid to evaluating and interpreting hematologic parameters in children and adolescents with obesity