A noninvasive surface wave method is proposed for measuring the complex modulus of viscoelastic tissues. In this method, a small harmonic force is generated by a mechanical shaker on the tissue surface. The vibration on the tissue surface is measured by an ultrasound transducer. The surface wave speed is calculated locally over a short distance by a phase gradient method. The viscous properties of tissue are calculated from the attenuation of the surface wave amplitude with distance. Together, the complex modulus can be derived. Experiments are conducted in a 10% gelatin phantom and a pork loin sample between 100-300 Hz. Experimental results show that for a 10% gelatin phantom, its storage modulus is around 10 KPa and its loss modulus is around 0.5 KPa, for the pork loin sample, its storage modulus is between 5-15 KPa and its loss modulus is in between 0.5-5 KPa. The amplitude of the complex modulus is independent of the frequency for the gelatin material, however it increases with the frequency for the pork loin tissue. The results indicate that the surface wave is much more dispersive in the pork loin sample than in the gelatin phantom. These results are consistent with previous findings. The proposed method does not depend on viscoelastic constitutive models and can be used for evaluating the tissue viscoelastic properties.