The vibration signal of fault gear is characterized by non-stationary and non-linear, and can be regarded as the superposition of some amplitude-modulation components and frequency-modulation components. The instantaneous frequency of a single component contains the fault characteristics of gear. In order to decompose the signal accurately and obtain the instantaneous frequency of a single component, a fault diagnosis method based on the local characteristic-scale decomposition (LCD) and the instantaneous frequency spectrum was proposed. Firstly,the original vibration signal was disposed by LCD, and the intrinsic mode components (ISC) whose instantaneous frequency have physical meaning were obtained.Secondly, each instantaneous frequency of ISC was computed by Hilbert transform (HT), and the instantaneous frequency spectrum was got by means of Fourier transform (FT). At last the instantaneous frequency spectrums were acquired to fault diagnosis. The fault data of gear broken teeth was analyzed. The analysis results show that the method can diagnose the fault of gear box, and locate the fault. The method can be applied to gear fault diagnosis effectively.