The rationality of tourists` expenditure decision making has been a controversy in the field of tourism and behavioral economics. Using a sample of tourists visiting Jeju island, this study examined whether tourists` expenditure decision making is rational or irrational. Particularly, this study separately examined each of the three different types of the tourists` consumption expenditures: food and beverage, transportation and lodging, and cultural and entertainment. Date were conveniently collected from 150 tourists of Jeju island at a rental car shop on June 2011. In order to test the rationality of the tourists`s expenditure, the respondents were asked about their expected and real expenditure on the three different types of expenditure before they rented a car and when they returned the car. Results showed that tourists consumption expenditure decision making could be characterized by both rationality and irrationality. Overall, the tourists were likely to spend their expenditure rationally. However, in terms of the entertainment & shopping items, the respondents were likely to spend their expenditure irrationally.