In this paper, we propose an ordinal optimization (OO) based algorithm for solving the resource allocation optimization problem of grid computing system to maximize the service reliability. An approximate model is firstly proposed to estimate the service reliability of a resource allocation design within a tolerable computation time. Next, we employ the proposed algorithm to solve the resource allocation optimization problem. The OO based algorithm consists of two stages. A binary particle swarm optimization (BPSO) algorithm is employed in the first stage using the approximate model for fitness evaluation and selects a subset of good enough solutions. Then, we proceed with the goal softening searching procedure in the second stage using more refined approximate models to search for a good enough solution. We have demonstrated the test results by simulating on an 8-node and 11-link grid computing system including one resource-managing node. The good enough solution obtained by the proposed algorithm is promising in the aspects of solution quality and computational efficiency. In addition, the proposed algorithm spends only 2.35 minutes in a Pentium IV PC to obtain the good enough resource allocation design.