An exemplary data center includes two server systems arranged back to back. Each server system includes a server cabinet, a rail system, at least two servers and a fan module. The servers are accommodated in the server cabinet. The fan module is located at a rear side of the two servers and perpendicular to the servers to provide a horizontal airflow to cool the servers. The fan module is capable of rolling on the rail system between a rear side and a front side of the server cabinet to interchange between a vertical state and a horizontal state. Assembly or disassembly of the fan module is performed from the front side of the server cabinet where there is a greater amount of workspace.