Outdoor education is mostly mentioned in terms of environmental education. The aim of this research is to determine the short term effectiveness of an outdoor environmental education program on biodiversity awareness, environmental awareness and sensitivity to natural environment. The data is collected from an outdoor environmental education project which is financed by TUBITAK and its name is "Nature-based Outdoor Environmental Education in Canakkale and Suburbs, 2008." There are 27 in-service teachers in the project and the project is only for 10 days. An environmental awareness and sensitivity scale is developed for the study. The research design is pre-posttest design. The data is analysed by Wilcoxon signed rank test because of the data is non-parametric. It is found that the nature-based outdoor education program is effective to improve environmental awareness and sensitivity to natural environment; on the other hand it is not successful to improve biodiversity awareness. [Note: The publication year shown on the PDF (2014) is incorrect. The correct publication year is 2015.]