In elastic optical networks (EON), spectrum fragmentation causes a large number of requests blocked. In order to decrease fragmentation and reduce the bandwidth blocking probability of immediate reservation (IR) requests, a two-dimensional fragmentation-aware routing and spectrum allocation (RSA) algorithm is proposed. In the routing phase, the proposed algorithm considers the route hop number and the modulation format, aiming to calculate multiple candidate paths with a smaller number of hops and a higher level of modulation format offline. In spectrum allocation phase, a two-dimensional fragmentation metric namely combined fragmentation metric (CF) is designed to evaluate each candidate spectrum block, which comprehensively considers three factors: distance to frequency boundary, impact on contiguous resources, and resource consumption. Then, the proposed algorithm will allocate the candidate spectrum block with the minimum CF to IR requests. The simulation results show that compared with several well-performed benchmark algorithms, the proposed algorithm can further reduce the bandwidth blocking probability and improve the spectrum utilization.