In 2016, Norway implemented an air passenger tax, the Flypassasjeravgift, as part of a national budget agreement. This thesis evaluates the effect of the tax on the number of international departing passengers from Oslo airport, Gardermoen. To estimate the causal effect, the synthetic control method (SCM) is applied, constructing a synthetic Oslo based on a weighted combination of comparable capital cities to approximate the evolution of departing passengers in the absence of the tax. The estimate suggests an average decrease of 76,547 passengers after the introduction of the tax, representing a 0.27% decrease. However, this estimated effect is not statistically significant, as similar or larger placebo effects are observed. Consequently, the thesis does not provide evidence that the Norwegian air travel tax reduced the number of international departing passengers departing from Gardermoen.