Abstract: Objective To discuss the optimal BMI boundary value points for thinness of Shanghai children and its corresponding percentiles. Methods (1)The data (height, weight and age) from 78458 children (39588 boys and 38870 girls) aged 2-18 years in Shanghai City were collected, including 23233 boys aged 2-11.5 years and 17742 girls aged 2-10 years. (2)Weight for height (WFH) for Shanghai children was used as the gold standard. Grade 1 slight thinness and Grade 2 middle and serious thinness were defined as those with weight below the 10th centile of WFH, and those with weight below the 2.28th centile of WFH (corresponding to z score<-2). (3)The Subject operating characteristic curve (ROC) was used to characterize the sensitivity and specificity of different BMI boundary value points in classifying underweight. Results (1) When classifying grade 1 thinness,the specificity on 10th,12.5th,and 15th centile of BMI is 98.7%,96.8% and 94.3% respectively for boys,and 99.4%,97.8% and 95.0% respectively for girls;the sensitivity on 10th,12.5th,and 15th centile of BMI is 89.2%,96.9% and 99.0% respectively for boys,and 85.3%,95.7% and 98.9% respectively for girls. (2) When classifying grade 2 thinness,the specificity on 3th, 4th and 5th centile of BMI is 99.3%,98.6% and 97.7% respectively for boys, and 99.5%,98.7% and 97.9% respectively for girls;the sensitivity on 3th,4th and 5th centile of BMI is 95.7%,99.6% and 98.6% respectively for boys,and 91.3%,98.4% and 99.5% respectively for girls. (3) The rate of grade 1 thinness (BMI < P12.5 and > P4) in Shanghai boys and girls aged 2-18yr is 7.9% and 7.5% respectively, and the rate of grade 2 underweight (BMIConclusion The optimal BMI boundary value points for grade 1 and grade 2 thinness is the 12.5th and the 4th centile of BMI.The corresponding BMI value for 18 years old is 18.2 and 16.9 kg/m² for boys,and 17.6 and 16.5 kg/m² for girls respectively.