Abstract: QoS routing problem is proved to be a NP-C problem, it is very difficult to solve the NP-C problem effectively by conventional routing algorithms. This paper puts forward an QoS unicast routing algorithm which is based on the principle of ant colony algorithm and used to solve the problem of bandwidth and delay constrain. The rule of ant colony algorithm that the ants find the shortest path through the laying down of pheromone and the rule with emphasis on the maximal network throughput and the lowest average cell delay are also used to define the transition probability of ants, routing table, and the way pheromone is updated. A detailed routing scheme algorithm based on the ant colony algorithm is implemented. Results prove that the algorithm has strong ability of searching an optimum solution and show its better flexibility and potential parallelism.