BiTiO3 of C1 C1 structure is found to be the most stable phase according to our first-principles calculations for nine possible structures, with corresponding optimized crystal parameters of a=b=5.606 Å, c=9.954 Å; α=β=105.1°, γ=61.2°. Subsequently, we have investigated the electronic and optical properties of BiTiO3 in C1C1 structure. It is found that BiTiO3 is a semiconductor with an indirect band gap with its energy band near Fermi level being dominated by O-2p and Ti-3d levels. Additionally, the dielectric function, refractive index, and reflectivity of BiTiO3 are also calculated, and it is shown that the optical properties of BiTiO3 are nearly isotropic.