Abstract: Objective: To compare the physical development of obese and normal boys aged from 0 to 5 years old to provide a basis for early identification and prevention of obesity. Methods: The diagnosis of childhood obesity was made using the WHO BMI standard of 2006 for children aged from 5 to 19 years old that BMI of xˉ+SD, xˉ+2SD, xˉ-2SD were defined as excess weight, obesity and malnutrition respectively. The children included in the analysis should meet the following three criteria at the same time: a. a 5-year-old boy who came to the Child Healthy Department of Chongqing Health Center for Women and Children for physical measurements from January 2008 to January 2020; b. taking physical measurements at all of the time points of 6, 12, 18, 24, 36, 48 and 60 months; c. those meeting the diagnostic criteria at the age of 5 without secondary obesity caused by endocrine diseases, drugs and tumors. The calculation formula of Z-score = (children observation value - mean of children of the same age and same sex)/standard deviation (SD). Results: There were 104 obese boys and 864 normal boys meeting the first inclusion criterion. Among them, 58 obese boys and 303 normal boys who met all of the three criteria were included in the analysis. The mean WAZ and BMIZ of the obese group were higher than that of the normal group from the age of 6 months, and the difference was statistically significant after the age of 18 months. The mean HAZ of the obese group continued to be higher than that of the normal group after the age of 18 months, and the difference was statistically significant after the age of 36 months. The mean ΔWt of the obese boys was consistently higher than that of the normal boys, and there was a statistically significant difference in the rate of weight gain after the age of 12 to 18 months. The mean of ΔHt in obese boys continued to be higher than that of normal boys after the age of 12 to 18 months, and there was a statistically significant difference in height increase between the age of 12 to 18 months and 36 to 48 months. Absolute mean value of ΔBMI of the obese boys from 6 to 12 months to 24 to 36 months were consistently lower than that of the normal boys, while those in the 36 to 48 months and 48 to 60 months were higher than that of the normal boy group. The difference of ΔBMI after 18 to 24 months was statistically significant. Conclusion: The prevention of childhood obesity should be before the age of 2 years and the age of 12 to 18 months was the first critical period. Obese children showed higher height and faster height growth in a short period of time, but there was no advantage in height growth after the age of 48 months.