Objective：To investigate the association of anteroposterior renal pelvic diameter（APD） measured by ultrasound and clinical data at initial visit with timing of surgical treatment in children with unilateral hydronephrosis due to ureteropelvic junction obstruc－tion（UPJO）. Methods：A retrospective analysis was performed for the clinical data of the children with unilateral hydronephrosis due to UPJO who were diagnosed in the outpatient service of Children’s Hospital of Chongqing Medical University from September 2013 to September 2016. The children divided into groups based on sex，affected side，age at initial diagnosis，and degree of hydronephrosis at initial diagnosis，and the chi-square test was used for comparison between groups（P<0.05）. A binary logistic regression analysis was performed for related factors，and a receiver operating characteristic（ROC） curve analysis was performed for APD and age at initial diagnosis. Results：There were 170 valid cases at the end of follow-up，and among these children，41（24.11%） chose surgical treat－ment. The rate of surgical treatment increased with the increase in APD at initial diagnosis（χ2=42.534，P=0.000），and there was no significant difference in the rate of surgical treatment between the children with different sexes or affected sides. The binary logistic regression analysis showed that APD at initial diagnosis（χ2=34.465，P=0.000） and affected side（χ2=4.067，P=0.044） were independent predictive factors for surgery during follow-up. At the optimal cut-off value of 1.75 cm，APD at initial diagnosis had an area under the ROC curve（AUC） of 0.894（95%CI=0.826-0.962，P=0.000），a sensitivity of 82.9%，and a specificity of 86.8%，and age at initial diagnosis had an AUC of 0.345（P=0.003）. Conclusion：Degree of hydronephrosis at initial diagnosis is an important factor for surgical decision-making for children with hydronephrosis due to UPJO. APD>1.75 cm at initial diagnosis has a certain value in predicting surgery during follow-up，and imme－diate pyeloplasty is recommended in case of APD>4 cm at initial diagnosis. Ultrasound examination at initial diagnosis has a guiding significance in the diagnosis and treatment of hydronephrosis and surgical decision-making.