Variations of electron traps in molecular-beam-epitaxial (MBE) GaAs layers grown on Si substrates by rapid thermal processing (RTP) have been investigated with deep level transient spectroscopy (DLTS). RTP was performed at 760 – 910 °C for 9s with Si02 encapsulant. In contrast with the layer on GaAs, the traps AI(Ec – 0.65eV) and A2(Ec – 0.81eV) are observed in the layer on Si. The trap EL2h, one of the EL2 family, is produced by RTP in the layer on Si. Some GaAs surfaces were etched to prove the deeper region. In the surface region, the concentrartion of EL2h is comparable to that of EL2 produced by RTP in the layer on GaAs. On the other hand, in ∿ 1 μm below the surface, the concentration of EL2h is about ten times as large as that of EL2. It is speculated that the stress from the GaAs/Si interface enhances the production of the EL2h concentration. In addition to the EL2, the traps R1(Ec – 0.23 eV), R2(Ec – 0.40 eV), R3(Ec – 0.43eV) and R4(Ec – 0.56 eV) are produced by RTP in the GaAs grown on Si.