We found experimental evidence for the first time that the in-plane strain affects the anisotropy of the hole subbands in the strained-Si pMOSFETs. The anisotropy was characterized by Hall factor, which was obtained by combining Hall and capacitance-voltage measurements. It was confirmed by Raman spectroscopy that the value of the strain in the strained-Si layer becomes larger with increasing Ge content x in the Si1-xGex buffer layer. At low temperature, Hall factors for x = 0.11, 0.14 and 0.19 devices exhibit almost the same values, which are larger than the values for the unstrained Si-pMOSFET. From the result, the lowest subband in the strained-Si device was found to be more isotropic than that in the unstrained-Si device. As temperature is raised up to 180 K, the Hall factor is almost constant for x = 0.19 device, whereas it decreases gradually for x = 0.11 device. The temperature dependence indicates that the lowest subband in the strained-Si device is more isotropic than the second subband. These results suggest that the lowest and the second subbands originate in the light- and the heavy-hole bands, respectively. This assignment is consistent with a recent calculation on two-dimensional hole gas in strained-Si pMOSFETs.