The potential energy surfaces of the ethylene radical cation near the 90 degrees-twisted form were clarified by an appropriate configuration interaction calculation. A characteristic conical intersection between the ground doublet and the first excited doublet states at a D-2d 90 degrees-twisted geometry was revealed. It was found that spin-charge separation occurs suddenly near the twist angle of 90 degrees. The effect of substitution on the shape of the potential energy surfaces is also discussed.