A high-quality thin Si layer on epitaxial spinel(MgO·Al2O3) on Si substrate has been developed. It was obtained by the solid phase epitaxial (SPE) regrowth of amorphous Si with Si seed islands on the epitaxially grown spinel. The SPE-Si layer on the epitaxial spinel was superior to conventional Si on sapphire (SOS) in stacking-fault density, Hall mobility, and contamination from the insulating materials. The SPESi layer has low stacking-fault density of 10–50 cm-2, even for 0.4 μm thickness. This value is eight orders of magnitude less than that of Si on epitaxial spinel obtained by vapor phase epitaxial (VPE) growth with SiH4. The SPE-Si also has high electron Hall mobility of 880 cm2/V-s for n=5×l016 cm-3, for 1-μm-thick Si. This value is about 85% of that in bulk Si, and is higher than that in Si on spinel grown by VPE-growth and SOS. (100) spinel layer was grown on (100) Si substrate by a MgCI2-Al-HCI-CO2-H2 VPE growth system. After thermal oxidation, Si seed islands were grown on the spinel by the pyrolysis of SiH4. Amorphous Si was deposited on the Si seed islands by chemical vapor deposition (CVD) of SiH4. The SPE-Si layer was obtained by regrowth of the amorphous Si in a hydrogen atmosphere.