A method and apparatus for acquiring system information (SI) in a wireless communication system is provided. When a user equipment (UE) moves to a (neighbor) cell and if the UE have stored system information with same system information area code (SAC), the UE reuses the stored system information (SI) and skip acquiring whole system information blocks (SIBs). If the UE does not have matched SAC, the UE checks existence of any delta SI scheduling configuration which means that a group of SI message is to be update based on the stored information. If a SI message supporting delta configuration is broadcast at a cell and if the SI message includes a parameter which UE has previously stored, the UE replaces the stored parameter by the parameter received from the SI message.