Taiwan's government enacted the Environmental Education Act in June 2011. In the beginning of the implementation of the Act, a national assessment of schoolteachers' environmental literacy was performed in order to establish the baseline for evaluating the effectiveness of environmental education policy. This large-scale assessment involved a large sample size of in-service teachers across schools at all levels and regions in Taiwan using the stratified random sampling method. Data showed that the teachers have satisfactory levels of environmental knowledge and attitudes, but presented low degrees of environmental action. Differing from previous research results, elementary teachers performed better than high school teachers on this assessment, which may be attributed to their teacher training experiences. Results from this study may provide implications for teacher professional development programs and school environmental education policy.