Provided is an availability prediction method for a high availability. The method includes calculating a basic survival probability that the other node survives until a failure on one node of two nodes constituting a cluster is fixed, and determining an optimal number of nodes meeting a preset reference availability probability by calculating an availability probability for a predetermined range of the number of nodes on the basis of the basic survival probability. The method determines the number of nodes in the high availability cluster so as to match a reference availability probability, and is able to accomplish an optimal configuration of a cluster by calculating the availability probabilities for combinations between active node and passive nodes and between head nodes and switches.