Interpretation has been considered as the ``soul`` of ecotourism due to its possible effectiveness on the visitors` environmental cognition, attitude, and behavior intentions; yet, to date, little research has empirically tested their effects. Therefore, the aim of this study was to empirically test the effects of the above environmental interpretations at a Korea national park on visitors`s cognition, attitude and behavior. Data were collected from 383 visitors of Jirsan National Park from August to October 2010. Interpretation Evaluation Tool Kit, developed by Ham and Weiler (2006), was used to measure the effects. The results showed that overall the interpretations made signigicant positive impacts on the visitors` cognition, attitude and behavior intentions. In particular, the respondents were most likely to agree that the interpretation programs made positive impacts on their attitude of environmental conservation, while they were least likely agree that the interpretation program influenced a positive impact on their personal value system. The results also showed that visitors` motivation might determine better than the length of interpretation time for the effectiveness of interpretations. Practical implications and future research direction were discussed at the end of the study.