Intrinsic carrier concentration (ni) is one of the most important physical parameters for understanding the physics of strained Si and Si1-xGex materials as well as for evaluating the electrical properties of Si-based strained devices. Up to now, the report on quantitative results of intrinsic carrier concentration in strained Si and Si1-xGex materials has been still lacking. In this paper, by analyzing the band structure of strained Si and Si1-xGex materials, both the effective densities of the state near the top of valence band and the bottom of conduction band (Nc andNv) at 218, 330 and 393 K and the intrinsic carrier concentration related to Ge fraction (x) at 300 K were systematically studied within the framework of KP theory and semiconductor physics. It is found that the intrinsic carrier concentration in strained Si (001) and Si1-xGex (001) and (101) materials at 300 K increases signiifcantly with increasing Ge fraction (x), which provides valuable references to understand the Si-based strained device physics and design.