A desired-speed car-following model is proposed in this paper. The driver's desired speed is considered so that the model can reflect the driver's characteristics and the difference between drivers. The model can explain why different drivers maintain different speeds or spacing under the same condition. The necessary and sufficient conditions for stability are discussed, and they indicate that if the driver's desired speed is close to equilibrium speed, traffic is stable. Otherwise, if the difference between the driver's desired speed and equilibrium speed is large, traffic may be unstable.