a method for transmitting security data in an ethernet pon system is provided . the method includes the steps of : a ) creating a frame comprised of a data field for encrypting the security data , a key information field for storing key information used for decrypting the encrypted data of the data field , and a security frame , wherein the security frame includes an onu id field for indicating onu id information identified by an onu having the destination user , and a user id field for indicating a security id identified by the destination user ; and b ) transmitting the created frame .