Objective: To develop an assessment tool for the prediction of postpartum blood loss over 300 mL and 500 mL after vaginal delivery. Methods: A retrospective case-control study of 504 (72 cases: 432 controls) pregnant women with gestational age of 28 weeks or above who underwent vaginal delivery at Chiang Rai Regional Hospital between 1 st October 2015 and 30 th September 2016 was conducted. The predictors were selected using "Sign OK" selection. The risk scoring system was developed according to the seven steps of clinical prediction model development of Steyerberg. Results: This form included eight predictors: maternal age of 35 years old or above, gestational age over 40 weeks, being nulliparous, history of curettage in prior pregnancy, gestational hypertensive disorder, hemoglobin level equal to or less than 10 g/dL, fundal height of 38 centimeters or above, and the use of pethidine for pain relief in the 1st stage of labor. The sensitivity of predicting postpartum blood loss over 300 mL after vaginal delivery by the risk scoring system was 80.7%, specificity of 60.8%, and the ROC curve was equal to 0.71 at the optimal cutoff score of four points or above. To predict the postpartum blood loss of 500 mL or above after vaginal delivery, the sensitivity was 88.1%, specificity of 54.6%, and the ROC curve was equal to 0.71 at the optimal cutoff score of three points or above. Conclusion: This form had an acceptable performance in predicting postpartum blood loss over 300 mL and 500 mL at the optimal cutoff scores of four points or above and three points or above, respectively.