The site symmetry of Yb3+ in In0.5Ga0.5P grown by liquid phase epitaxy has been investigated using a polarized luminescence spectroscopic technique. The site symmetry of indium in In0.5Ga0.5P was lowered from T-d to C-2v resulting from substitution of indium by ytterbium. The lowering symmetry was confirmed by the spectroscopic observation of the splits of Gamma(8) levels of T-d in C-2v symmetry. The substitutional site symmetry of Yb3+ in In0.5Ga0.5P, i.e. Yb-In(3+), was revealed as C-2v.