Recently, Cloud computing has emerged as a new computing paradigm on the Internet. Cloud computing facilitates flexible and efficient computer resource management via virtualization technologies at anytime and from anywhere, so that users can add and/or delete IT resources. Users can set up and boot the required resources and they have to pay only for the required resources. However, they have to spend a considerable amount of time and money to design, set up, boot, and monitor their resources. Thus, in the future, providing a mechanism for efficient resource assignment and management will be an important objective of cloud computing. In this paper, we propose a dynamic resource allocation method based on the load of VMs on IaaS, abbreviated as DAIaS. This method enables users to dynamically add and/or delete one or more instances on the basis of the load and the conditions specified by the user. We implement a prototype to evaluate the effectiveness and efficiency of DAIaS. Furthermore, we perform an experiment to extract the prototype on a real cloud service, namely, Amazon EC2.