The production of greenhouse has become an important means to increase the income of China's rural areas and farmers, because the environment of greenhouses is controllable and the production is efficient. The temperature and humidity are the most important environmental impacts in the production of crop. The traditional regulation of the environment in greenhouses relies on artificial sampling detection which is inefficiency. Some advanced greenhouses use field wiring to implement the environmental regulation, but the environment is complex so field wiring is also a difficult problem. Aiming at solving the problem of monitoring the temperature and humidity in greenhouses, this paper designs a on-line monitoring system of greenhouses which is based on the wireless communication technology. The system adopts the DHT11 sensors to monitor the temperature and humidity in the greenhouse, and adopts the wireless communication technology of ZigBee so as to realize the online monitoring of the temperature and humidity. By using the wireless transmission and data acquisition nodes which are placed in different greenhouses can transmit the value of the temperature and humidity in greenhouses to the central node easily, and also realized the function of the online monitoring, alarm and transmitting data from different data acquisition nodes to the central node. The system consists of a data acquisition module, a wireless communication module, an alarm module and so on. The system is advanced because of its simple structure, small volume, low cost, easy operation, wide application range, strong scalability and so on. Compared with the traditional system, it can meet the demand of acquiring the temperature and humidity value of greenhouses in China.