A method of performing cell reselection by a mobile station which camps on a serving cell of a first radio access technology, RAT, in a wireless communication system, comprising: determining whether first priority information for the serving cell of the first RAT is provided by the serving cell; receiving, from the serving cell, second priority information for a second RAT; if the first priority information for the serving cell of the first RAT is provided, performing inter-RAT cell reselection based on the first and second priority information; and if the first priority information for the serving cell of the first RAT is not provided, performing cell reselection toward the second RAT based on measured values of cells of the second RAT without considering the second priority information, wherein: - the first RAT is GSM EDGE Radio Access Network, GERAN, and the second RAT is UMTS Terrestrial Radio Access Network, UTRAN, - the first priority information includes a priority and a threshold for the serving cell of the first RAT used for inter-RAT cell reselection, and the second priority information includes a priority and thresholds for the second RAT used for inter-RAT cell reselection, and - the step of performing cell reselection toward the second RAT based on measured values of cells of the second RAT includes: -- determining at least one suitable cell used for reselection from the first RAT to the second RAT, wherein a measured value of each of the at least one suitable cell of the second RAT exceeds a measured value of the serving cell by an offset during an interval; and -- reselecting a cell with the greatest measured value among the at least one suitable cell of the second RAT. determining whether first priority information for the serving cell of the first RAT is provided by the serving cell; receiving, from the serving cell, second priority information for a second RAT; if the first priority information for the serving cell of the first RAT is provided, performing inter-RAT cell reselection based on the first and second priority information; and if the first priority information for the serving cell of the first RAT is not provided, performing cell reselection toward the second RAT based on measured values of cells of the second RAT without considering the second priority information, wherein: - the first RAT is GSM EDGE Radio Access Network, GERAN, and the second RAT is UMTS Terrestrial Radio Access Network, UTRAN, - the first priority information includes a priority and a threshold for the serving cell of the first RAT used for inter-RAT cell reselection, and the second priority information includes a priority and thresholds for the second RAT used for inter-RAT cell reselection, and - the step of performing cell reselection toward the second RAT based on measured values of cells of the second RAT includes: -- determining at least one suitable cell used for reselection from the first RAT to the second RAT, wherein a measured value of each of the at least one suitable cell of the second RAT exceeds a measured value of the serving cell by an offset during an interval; and -- reselecting a cell with the greatest measured value among the at least one suitable cell of the second RAT. - the first RAT is GSM EDGE Radio Access Network, GERAN, and the second RAT is UMTS Terrestrial Radio Access Network, UTRAN, - the first priority information includes a priority and a threshold for the serving cell of the first RAT used for inter-RAT cell reselection, and the second priority information includes a priority and thresholds for the second RAT used for inter-RAT cell reselection, and - the step of performing cell reselection toward the second RAT based on measured values of cells of the second RAT includes: -- determining at least one suitable cell used for reselection from the first RAT to the second RAT, wherein a measured value of each of the at least one suitable cell of the second RAT exceeds a measured value of the serving cell by an offset during an interval; and -- reselecting a cell with the greatest measured value among the at least one suitable cell of the second RAT. -- determining at least one suitable cell used for reselection from the first RAT to the second RAT, wherein a measured value of each of the at least one suitable cell of the second RAT exceeds a measured value of the serving cell by an offset during an interval; and -- reselecting a cell with the greatest measured value among the at least one suitable cell of the second RAT.