Abstract: A method for and an apparatus for controlling and optimizing a xylene isomer separation and isomerization process, in which near infrared light of a wavelength ranging from 1,100 nm to 2,500 nm is transmitted through samples obtained at different stages of the process from raw materials flowing in the process by use of an analyzer system using optical fibers, thereby simultaneously measuring, in an on-line manner, xylene isomers and aromatic hydrocarbons containing 6 to 9 carbon atoms from those samples. Accordingly, it is possible to monitor the entire operation state of the process within 15 minutes in an on-line manner. Conventionally, 2 to 3 days are taken for the same monitoring.