Cloud computing is a distributed architecture that has shared resources, software, and information is provided to computers and other devices on a scalable platform and demand. Availability of the cloud services is one of the key security issues in it. Distributed Denial of Service (DDoS) is an attack that threats the availability of the cloud services. In this paper, effect of the DDoS attack on the cloud is investigated. Therefore, a model for attack based on the DDoS is designed, and then the authors simulate a cloud system on the experimental environment. Experiments show that the cloud system is vulnerable to this attack.