Abstract: A speech coding algorithm applied to narrowband transmission at very low bit rate is proposed. Based on the MELP standard working at 2.4kbps, joint vector quantization is applied to linear spectrum pair’s parameters, interpolation and nonlinear quantization are applied to pitch, gain parameters are compressed efficiently and auditory perception is considered, speech data could be coded and transmitted smoothly at 0.5kbps The predicative residuals of linear spectrum pair’s parameters are also used for vector quantization; it’s a suitable way of improving synthetic speech quality. The experimental results reveal that the proposed algorithm can ensure speech effective transmission at very low bit rate while the intelligibility of received speech is acceptable.