Partly due to the climate changes, flooding occurs more often than before. Particularly high urbanization causes more damages by flooding with unexpected downpours. Recent mobile telecommunication technology can help to reduce the damage by a natural disaster with quick alarming process. This research aimed to implement a flood warning GIS service system based on open source software and mobile hybrid application program with LBS. The developed system utilized location information of mobile clients for smart phone users to get alerted to flooding immediately. This service system would be very useful in urban areas to reduce the flood damages.