A cooling system for power modules is provided. The cooling system includes two covers, a plurality of power modules and a plurality of first spaces. The power modules are disposed between the two covers. Each power module includes a housing, a circuit board and heat dissipation elements disposed on the two sides of the circuit board. There is a through hole on the housing. Each first space is formed between two neighboring power modules or is formed between the cover and the neighboring power module. The heat dissipation elements of each power module are located in the neighboring first spaces, and the through hole of each power module is in communication with the neighboring first spaces. The first spaces and the through holes of the power modules are communicated with each other to form a coolant passageway collaboratively for allowing a coolant to pass through.