Nowdays, many cloud services are provided over the OpenStack platform. How to improve the network performance of OpenStack has become an important topic. In this paper, we propose a method to mitigate network congestion in OpenStack. In our method, when network congestion occurs, a VM (virtual machine) live migration design and a flow path adjustment design are used with the SDN (software-defined networking) technology to mitigate network congestion. We performed several experiments under the fixed-rate flows and the unlimited-rate flows conditions. Then, we compared the results of our proposed method with the results achieved by using the ECMP (Equal Cost Multiple Path) routing method. We show that our proposed method reduces total file transer time in an OpenStack network and performs better than ECMP. In addition, we found that the VM live migration design is more effective in mitigating network congestion than the path adjustment design.