Measurements of cell mechanics are crucial not only for understanding various cell behaviors such as migration and proliferation but also for distinguishing different types of cells. It has been reported that the stiffness of cancer cells is smaller than that of normal cells [1, 2]. However, it is little known how rheological properties of cells differs between normal and cancer cells. In this study, we measured the complex shear modulus, G∗, of human mammary epithelial cells (MCF-10A) as normal cell and human mammary adenocarcinoma cells (MCF-7) as cancer cell by atomic force microscopy (AFM) with microarray technique [3, 4], in a frequency range of 2-180 Hz. It was observed that G∗ of both types of cells followed a power-law rheology [5]. We found that the elastic modulus at a frequency of the normal cells was significantly larger than that of the cancer cells, which was consistent with that reported previously [6]. Moreover, the power-law exponent and Newtonian viscous damping coefficient also had significant difference between the normal and cancer cells, suggesting that the set of parameters of power-law rheology is a useful indicator for distinguishing normal and cancer cells. We will present in detail how the ensemble distribution of power-law rheology of normal and cancer cells is affected by the modification of cytoskeletal structures.[1] J. Guck, et al., Biophys. J. 88, 3689 (2005), [2] S. E. Cross, et al., Nat. Nanotech. 2, 780 (2007), [3] Y. Mizutani, et al., Jpn. J. Appl. Phys. 47, 6177 (2008), [4] S. Hiratsuka, et al., Ultramicroscopy 109, 937 (2009), [5] B. Fabry, et al., Phys. Rev.E, 68, 041914 (2003), [6] S. Park, et al., Biophys. J. 89, 4330 (2005)