Provided is a method and apparatus for encoding and decoding an enhancement layer to reduce quantization error in a G.711 codec. Exponent indices of additional mantissa information of each sample are calculated based upon exponent information of each sample in a frame. A process of allocating 1 bit to each sample with a current exponent index is repeated, the exponent index starting from the maximum value while decreasing by 1 at every repetition until the total number of bits allocated to the samples is equal to the total number of available bits in the frame. And the most significant bits, as many as the number of bits allocated to each sample, are extracted from the additional mantissa information of each sample in the frame.