An apparatus for monitoring extinction ratio (ER) of optical signals comprises an optical spectrum analyzing unit, an ER monitoring control unit and an ER monitoring output unit. The optical spectrum analyzing unit measures two peak values corresponding to level one and level zero of optical signals from an optical coupler, and obtains two wavelengths for the two peak values. The ER monitoring output unit outputs the difference of the two wavelengths to the ER monitoring control unit. With a relation formula of the wavelength difference, the ER monitoring control unit estimates an optimal resolution bandwidth for setting up the optical spectrum analyzing unit. As such, the optical spectrum analyzing unit measures two optical powers corresponding to level one and level zero of optical signals. With the two optical powers, the ER monitoring output unit computes an ER value.