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A good cantilever should possess a small minimum detectable deflection (MDD), which depends on the measurement sensitivity and noise. Although increasing the measurement sensitivity can decrease the MDD of a cantilever, the noise will be a mainly limited factor when the measurement sensitivity is further increased. Therefore, the noise analyses of a piezoresistive cantilever are very important in improving cantilever resolutions. In this paper we analyze the noise characteristics of three silicon materials based on a large number of noise measurement results, which show that the 1/f noise is the dominant noise source at low frequencies and noise at high frequencies is Johnson noise. With the linear relation between 1/f noise and 1/L (piezoresistor length), Hooge factor α of three materials was calculated. Optimized piezoresistor dimensions for a piezoresistive cantilever are given at last.