An apparatus and method of performing cell reselection in a wireless communication system is provided. A mobile station determines whether first priority information for the first RAT is provided by the serving cell, and receives second priority information of a second RAT from the serving cell. The mobile station performs inter-RAT cell reselection based on the first and second priority information if the first priority information for the first RAT is provided. The mobile station performs cell reselection toward the second RAT based on measured values of cells of the second RAT without considering the second priority information if the first priority information for the first RAT is not provided.