A Mobil Ad hoc Network (MANET) is a dynamic multi-hop infrastructure-less wireless network that comprises of a collection of mobile, self-organized, self-configured wireless nodes. A routing protocol in MANET is used to find routes between mobile nodes to facilitate communication within the network. Numerous routing protocols have been proposed for MANET. Those routing protocols are designed to adaptively accommodate for dynamic unpredictable changes in network's topology. MANET characteristics such as openness, restricted resources and decentralization impact node's efficiency and made it easy to be affected by various security attacks, especially the Distributed Denial of Service (DDoS) attacks. These DDoS attacks consume all system resources like battery power, bandwidth, energy, CPU, and render those resources and nodes unavailable to the legitimate users. The aim of this research is to implement a simulation model called Distributed Denial of Service Simulation Model (DDoS-Sim) to examine the effect of DDoS attack on various routing protocols in MANETs namely: Ad hoc On-Demand Distance Vector (AODV), Dynamic Source Routing (DSR) and Destination-Sequenced Distance-Vector (DSDV). These routing protocols follow the reactive and proactive routing algorithms. The introduced model uses the network simulator 2 (NS-2) to apply DDoS on the three selected routing protocols. The performance of routings protocols was analyzed under the consequences of the attack in terms of packet delivery fraction and end-to-end delay.