Cloud computing is providing many services in our daily life. When deploying virtual machines in a cloud environment, virtual machine placement strategies can significantly affect the overall security risks of the entire cloud. In recent years, some attacks are specifically designed to collocate with target virtual machines in the cloud. In this paper, we present a Security-aware Multi-Objective Optimization based virtual machine Placement algorithm (SMOOP) to seek a Pareto-optimal solution to reduce overall security risks of a cloud. SMPPO also considers resource utilization on CPU, memory, disk, and network traffic using several placement strategies. Our evaluation results show that security of clouds can be effectively improved through virtual machine placement with affordable overheads.