Although global commercialization of transgenic plants has rapidly increased in the past 20 years, biosafety issue is still a constraint on further expansion and use of transgenic plants. Risk assessment has always been performed before commercialization of transgenic plants, but risk management measures, including environmental monitoring, are still necessary to ensure the safe application of transgenic plants. After 20 years of extensive application, some environmental risks caused by transgenic plants have gradually been revealed, such as resistance of target organisms, impacts on biodiversity, gene flow, and long-term persistence of transgenic plants in ecosystems. This reminds us that risk assessment is not enough to guarantee the safe application of transgenic plants; systematic and long-term environmental monitoring is necessary to identify the actual environmental effects. Laws and technical guidelines on environmental monitoring of transgenic plants have been issued by international organizations, such as the United Nations Environment Program (UNEP) and the European Union. Some countries have already performed systematical environmental monitoring of transgenic plants. This article summarizes the environmental risks of transgenic plants and the contents of environmental monitoring.