A system for monitoring an optical output/wavelength is employed to be used for a WDM system having a narrow channel space by structuring an etalon and photodiode as an integrated structure. The system includes: a laser source control unit for controlling the laser source; an optical/wavelength monitoring unit for monitoring an optical output/wavelength of the controlled laser source; a TEC control unit for controlling a TEC in order to constantly maintain the laser source of the optical output/wavelength monitoring unit to have a predetermined temperature; a temperature control unit for controlling a heater and a thermistor to set an etalon to a predetermined temperature, wherein the heater is attached on the optical output/wavelength monitoring unit and the thermistor is attached on the heater; a comparison unit for comparing the optical output signal and the wavelength signal, each of which is monitored by the optical output/wavelength monitoring unit; and a processing unit for comparing values of the compared signals with a preset value to control an input current or a temperature of the laser source.