With the rapid development of sensor technologies, in nowadays modern cities are deployed with various types of sensors to gather the environmental data. It is without doubt that how to deal with the huge amount of data collected by various sensors in an efficient way and to transform these data to useful information for the citizens to make use of has become an important research topic in building intelligent cities. The vast amount of data produced around us such as the temperature, the road conditions and the air quality can be numerically analyzed by utilizing the cloud computing technology. In this research, we implemented a MapReduce-based sensor data processing platform for intelligent cities. Specifically, we focus on using the MapReduce framework to process the raw data uploaded from the sensors, and then using HBase, which is a distributed, scalable, big data store, to save the sensor data. Besides, we use the Hadoop Distributed File System (HDFS) to store the street images captured by event data recorders installed in vehicles. In sum, the data processing platform we developed can be an important building block for constructing various useful applications to serve the citizens in intelligent cities.