A new technique namely chemical bonding by surface modification (CBSM) has been developed to bond GaAs thin films on Si substrate. The bonded GaAs epilayer, detached from its orginal substrate in conjunction with epitaxial lift-off (ELO) process, possessed smooth surface morphology and exhibited uniform bonding across the interface. The X-ray full width at half maximum (FWHM) of 16 are-sec from double crystal X-ray diffraction (DCD) and no peak wavelength shift from photoluminscence (PL) measurements reflected the high crystalline quality of the bonded films. The depth profile by XPS revealed at the interface the occurrence of surface modification. The surface modification resulted in the formation of Ga2Se3 and SiS2, which attributed for the strong and high quality bonding between GaAs and Si.