Ab initio complete active space self-consistent field calculations have been performed to determine the conical intersection (CIX) of styrene where the radiationless relaxation into S-0 takes place. The CIX is a crossing region between zwitterionic and diradical covalent states. In the zwitterionic state. the electron is partially transferred from the methylene into the benzylidene group. It has been also examined how the truncation of the active space affects the optimized geometry of the CIX