A fuel-saving path planning navigation system and a fuel-saving path planning method thereof. A plurality of sensors monitors a plurality of roads respectively and periodically report traffic information to a server. The server comprises a storing module and a processing module. The storing module stores the traffic information as history information. The processing module estimates how many vehicles on a road based upon the history information, calculates fuel consumption of each possible planned path including current fuel consumption based on real-time traffic and future fuel consumption based on the historical information, and plans a most fuel-efficient path. Also, the processing module recalculates fuel consumption of remaining path and changes the remaining path while fuel cost of a new path better 10% than the remaining path.