A RACH-related system resource optimization method and apparatus is provided for a wireless communication system. Specifically, a method is provided in a user equipment for optimizing a system resource related to a random access channel in a wireless communication system. A downlink channel condition and a transmission capability for performing random access to a base station based on a stored message size parameter are measured. The measured downlink channel condition and the measured transmission capability are reported. When a new message size parameter determined based on the reported downlink channel condition and the reported transmission capability is received, the stored message size parameter is updated with the new message parameter.