As an emerging technology, software defined networking (SDN) allows flexible control of network devices and supports user applications with guaranteed quality of service (QoS). To achieve flow transmission between two non-adjacent switches in SDN, efficient routing algorithm should be designed. In this paper, we jointly consider route selection and flow allocation problem. To stress the service sensitivity on delay, we design an optimization problem which minimizes the total delay subject to data transmission and service requirement constraints of user flow. Since the formulated optimization problem is an NP-complete problem, we propose a modified minimum cost algorithm. By selecting the minimum cost pathes in an updated auxiliary network iteratively, the optimal joint routing and flow allocation strategy can be obtained. Numerical results demonstrate the effectiveness of the proposed algorithm.