Two complex molybdenum oxides, Na 5 Ln(MoO 4 ) 4 (Ln=Gd, Er), have been prepared under high temperature solid-state reactions. The synthesized compounds have been investigated by means of the single-crystal X-ray diffraction and spectral measurements. All of the investigated structures crystallize in space group I4 1 /a with Z=4; a=11.4720(3)Å, R 1 (residual factor for all date)=0.0548 for Na 5 Gd(MoO 4 ) 4 ; a=11.3745(3)Å, R 1 (residual factor for all date)=0.0514 for Na 5 Er(MoO 4 ) 4 . Additionally, the calculations of band structure, density of states, dielectric constants, and refractive indices were performed with the density functional theory method for Na 5 Gd(MoO 4 ) 4 . The results show that the solid-state compound of Na 5 Gd(MoO 4 ) 4 is an insulator with direct band-gap of 3.695eV, and the calculated refractive indices is 2.063 at the static case.