Introduction and purpose: Influenza viruses can show resistance to anti-viral drugs.This can result from over-use of these drugs and also due to the ability of influenza virus to mutate rapidly.For this reason, new antiviral drug was made to treat influenza virus that is tolerated.Pre-clinical study and phase Ⅰ clinical trial were performed to investigate the drug effect and safety.Therefore, the purpose of this study was to select dosage of this new antiviral drug for phase Ⅱ clinical trial through population pharmacokinetic approach.Methods: Pharmacokinetic model of phase Ⅰ clinical trial was built using NONMEM (ver 7.3) and evaluated using goodness of fit (GOF) and visual predict check (VPC, n=1000).Monte Carlo simulation (n=1000) was performed to simulate pharmacokinetic data for 40mg/kg, 60mg/kg, and 80mg/kg dosage.Non-compartmental analysis (NCA) of pre-clinical study data and simulated data was performed using WinNolin (ver 6.4).To compare Cmax and AUC from pre-clinical study groups showing therapeutic effect and simulated groups with a range of dosage, a numeric prediction check (NPC) was performed using R program.Results: A range of Cmax and AUC was defined from the pre-clinical groups showing therapeutic effect.Simulated pharmacokinetic data from the validated population pharmacokinetic model was used to predict the Cmax and AUC for dosage of 40mg/kg, 60mg/kg, and 80mg/kg.NPC results showed that the simulated groups with dosage of 40mg/kg, 60mg/kg, and 80mg/kg had 60%, 98.5%.75.5% and 62.93%, 57%, 23% of the individuals within the therapeutic range of Cmax and AUC, respectively.Conclusion: Based on population pharmacokinetic analysis of phase Ⅰ clinical data and NPC results comparing the pharmacokinetic parameters of preclinical and simulated data, 40mg/kg to 60mg/kg dosage are recommended for phase Ⅱ clinical trial.Selection of dosage is reasonable since it is based on population approach of phase Ⅰ clinical trial and its comparison with pre-clinical study.