Abstract: Provided is a system for customizing a weapon in the online first person shooter game provided by a provider in which a plurality of users participate through computer terminals of the users connected through Internet and the plurality of the users form teams to compete, the system including: a game content providing server for providing the online first person shooter game contents to the computer terminals through the Internet network; and a weapon customization server for generating a gun from among a plurality of guns in a computer terminal in order to perform a mission of the game, determining whether or not the gun is customizable according to a gun customization request from the computer terminal, and performing the gun customization by mounting custom parts selected by the computer terminal on the gun when the gun is customizable according to a result of the determination.