Background: The standard treatment of Kawasaki Disease (KD) is intravenous immunoglobulin (IVIG) 2g/kg and aspirin. However, the most effective dosage of IVIG to prevent coronary artery abnormalities (CAA) is not known. The purpose of this study was to identify the dose of IVIG for KD patients in acute phase using a national inpatient database in Japan. Method: We used the Diagnostic Procedure Combination database to identify KD patients treated with IVIG between 2010 and 2017. We used multiple logistic regression models to estimate the risk of proportion of CAA and IVIG resistance, and multiple regression models to estimate the difference of length of stay and medical cost. In all models, demographic characteristics and comorbidities were adjusted for as covariates. We generated restricted cubic splines models with the same covariate specification as the fully adjusted model with five knots to examine non-linear associations between IVIG dose and the different outcomes. Results: We extracted 42,331 eligible patients from the database. The age was 1.7 years (1.6 SD), male 24432 (58%), and body weight 11.1kg (4.9 SD). The mean dose of IVIG was 1.7g/kg (1.3 SD), proportion of CAA 1325(3.1%), and medical cost 6596 USD (5160- 8689). We found a reverse J-shaped association between the dose of IVIG and the proportion of CAA. IVIG dosage was also found to have U-shaped associations with length of stay and medical costs. Conclusion: When IVIG dose was less than 2g/kg, the proportion of CAA increased, and the length of stay and medical cost also increased; when IVIG dose was more than 2g/kg, the length of stay and medical cost could be increased, and 2g/kg IVIG was considered appropriate for the initial treatment of KD.