Molecular beam epitaxy (MBE) grown GaAs films on Si substrates (001) 4 degrees off towards [111] A and towards [111] B, were examined by means of transmission electron microscopy (TEM). The results indicate that in both samples, threading dislocations in the GaAs epilayer are blocked mainly in a thin layer near the GaAs-Si interface. This thin layer is like an inner interface, consisting of pyramidal islands and is flatter on the As growth surface than that on the Ga growth surface. In the type B sample, the density of dislocations is lower, the inner interface is flatter and the number of twins is much larger than that in the type A sample.