A new phase of C2/m Ge16 is first proposed in this paper. The structures and mechanical, anisotropic, electronic, transport and optical properties of Si-Ge alloys in the C2/m phase are studied using first principles calculations. All Ge16 and Si16-x Ge x alloys in the C2/m phase are proven to have mechanical and dynamic stability. By analyzing the three-dimensional (3D) perspective of the effective mass and Young's modulus, obvious anisotropies of transport and mechanical properties are found. Higher-resolution full band structures are obtained to determine the positions of the valence band maximum (VBM) and conduction band minimum (CBM). All materials have a higher photoelectron absorption than that of diamond Si. A high electronic mobility (16 527 cm2 V-1 s-1) and hole mobility (3033 cm2 V-1 s-1) are found in C2/m Si8Ge8 and Si4Ge12, respectively. Based on the large mobility and photoelectron absorption, the Si-Ge alloys in the C2/m phase are promising materials for electronics and optoelectronics applications.