Wavelet transform decomposes signals in both time and frequency domains. According to the principle of multi-resolution, irregularities or complexities of decomposed signals in various frequency bands are different, which can indicate mechanical fault. Fractal dimension of decomposed signal is an excellent factor to express the irregularity or complexity. It can make features of mechanical faults remarkable. Based on wavelet fractal dimension, successful diagnosis of looseness fault of bearing bridge for 50MW turbo-generator was obtained.