Compliant substrates are one of the many promising lattice engineering approaches for hetero-epitaxy. There are many approaches to realize such compliant substrates. Silicon-on-insulator (SOI) wafer is a good option as it is readily available from industry. Previous studies on SOI using silicon thicker than 10 nm on insulator reported limited or no compliance. However, the effect of ultrathin silicon thickness (< 10 nm) on the compliance of SOI wafer has not previously been studied. In this work, we investigate the quality of GaAs hetero-epitaxial films on SOI as a function of ultra-thin Si layer thickness. Results show that the quality of GaAs films deposited by MetalOrganic Chemical Vapour Deposition improves with decreasing Si thickness. When Si thickness is < 4 nm, the resultant GaAs film is single-crystalline. When Si thickness is > 4 nm, the GaAs film is multi-crystalline. These films have been characterized by X-ray diffraction measurement, atomic force microscopy and transmission electron microscopy. These findings show the potential for SOI wafers as compliant substrates at ultrathin Si thicknesses.