A new earthquake precursor called ''coda Q'' is demonstrated for the Tangshan earthquake of 1976 and for the Haicheng earthquake of 1975. The precursor is based on the measurement of coda decay rate for small local earthquakes in the general area that is preparing for a major earthquake. During the 3-year period preceding the Tangshan earthquake the coda Q was about 3 times lower than it was before or after this period. A comparable change also occured at the time of the Haicheng earthquake. The observed change cannot be due to the change in the source parameters, such as the focal depth or the source spectrum. The coda Q precursor appears to detect the same physical process detected by other precursors such as b value, quiescence, radon, water quality, Vp/Vs, and animal behavior but has advantages over them in its clearer physical mechanism, easier sampling, and better defined anomalous region.