The provided forward error correction (FEC) apparatus for a transmission system includes a low pass filter (LPF) passing only a low frequency component of an input signal having a predetermined transmission rate; an AC power meter converting an energy value of an signal output from the LPF into a DC voltage value; a logarithm amplifier converting the exponentially varying DC voltage value from the AC power meter into a linear logarithmically varying DC voltage value; a digital signal processor (DSP) determining the transmission rate of the input signal according to a magnitude of the linear DC voltage value and outputting a control signal according to the transmission rate; a clock generator receiving the control signal and generating a clock signal according to the control signal; and a FEC module receiving the clock signal and performing FEC operation on the received signal. A method for FEC is also provided.