Abstract: A speech signal encoding/decoding method is provided. The method of encoding LPC coefficients includes dividing the nth-order line spectral frequencies into lower, middle and upper code vectors, quantizing the middle code vectors using a middle code book to generate a first index, selecting one of a plurality of lower code books according to the lowermost line spectral frequency of the middle code vector and the line spectral frequencies of the lower code vectors, and quantizing the lower code vectors using the selected lower code book to generate a second index, selecting one of a plurality of upper code books according to the uppermost line spectral frequency of the middle code vector and the line spectral frequencies of the upper code vectors, quantizing the upper code vectors using the selected upper code book to generate a third index, and transmitting the first, second and third indexes.