A method and apparatus for performing a conditional mobility based on a congestion level in a wireless communication system is provided. That is, the congestion level may be considered as mobility triggering condition of the conditional mobility. A wireless device performs a congestion level measurement, and receives a conditional mobility configuration associated with multiple target cells from a serving node. The conditional mobility configuration includes a threshold for the congestion level. The wireless device performs a mobility towards one target cell among the multiple target cells based on a result of the congestion level measurement and the threshold for the congestion level.