The disclosure relates to an optical receiver having an apparatus for varying decision threshold level and an optical transmission system having the same. An optical receiver having an apparatus for varying decision threshold level includes a photo diode (PD) for receiving an optical signal and transforming the received optical signal into an electrical signal; a trans-impedance amplifier (TIA) for pre-amplifying the electrical signal transformed by the PD; a limiting amplifier (LA) for deciding the electrical signal amplified by the TIA as either level 0 or level 1 and for amplifying the decision signal; a clock and data recovery (CDR) for generating a clock and data from the amplified decision signal by the LA; and a control circuit for adjusting a decision threshold level depending on the received optical signal power by the PD and for providing the adjusted decision threshold level with the LA.