Disclosed herein is a quantization method and apparatus of an audio encoder. The quantization method comprises calculating an absolute value of a maximum frequency spectrum of a first frame, externally received, by analyzing frequency spectrum data of the first frame, setting an initial value of a common scale factor to be used to quantize the first frame based on the absolute value of the maximum frequency spectrum of the first frame and an absolute value of a maximum frequency spectrum of a second frame, which has previously been calculated, and quantizing the frequency spectrum data of the first frame based on the set initial value of the common scale factor. Accordingly, before quantization is performed, an initial value of a common scale factor which is almost close to a value of an actual common scale factor can be previously set.