Ultrasound modulated optical tomography combines the advantages of light imaging with ultrasound and is able to image objects embedded in turbid medium. It is a promising technique. A relation between the modulated optical signal and the optical parameters of tissue inside and outside of the ultrasound zone is given by experiment. It is discovered that the modulation depth is directly relevant to the optical properties of the tissue at the ultrasound focal zone. The modulation depth reduces as the scattering coefficient of sample increases, and increases with the increase of absorption coefficient. The modulation depth of signal in ultrasound zone is not subject to the optical properties of complex tissue outside of the ultrasound zone. All these results will provide necessary basis for the image reconstruction of ultrasound modulated optical tomography.