One of the main characteristics of Ad hoc networks is node mobility,which results in constantly changing in network topologies.Consequently,the ability to forecast the future status of mobility nodes plays a key role in QOS routing.We propose a random mobility model based on discretetime Markov chain,called ODM.ODM provides a mathematical framework for calculating some parameters to show the future status of mobility nodes,for instance,the state transition probability matrix of nodes,the probability that an edge is valid,the average number of valid-edges and the probability of a request packet found a valid route.Furthermore,ODM can account for obstacle environment.The state transition probability matrix of nodes can quantify the impact of obstacles.Several theorems are given and proved by using the ODM.Simulation results show that the calculated value can forecast the future status of mobility nodes.