Two alternative orientations of a GaAs layer in respect to a vicinal (001) substrate, misoriented toward [110], are possible for most of the miscut angles; type A orientation having the GaAs [110] parallel to the [110] Si misorientation direction and type B with the GaAs [110] parallel to the [110] Si direction. The dependence of the GaAs surface roughness and of the tilting between the GaAs and Si (001) planes on the GaAs/Si orientation and the miscut angle of the vicinal substrate, in the range of 0-9 o , has been investigated. The GaAs/Si surface roughness was characteristic for preferential growth along [110]. Thin GaAs films of approximately 2μm thickness exhibited an almost constant rms roughness of 6.5-7.0nm for the type A orientation, while the rms roughness varied in the range of 4.2-60nm for the type B orientation, depending on the value of the miscut angle. The tilting angle between the GaAs and Si (001) planes exhibited different signs for the two types of orientation and much higher values for the type B orientation. A rather constant negative tilting was observed for type A samples, in the range of miscut angles of 1.5-7.5 o . The results suggest that the GaAs/Si lattice tilting may be a sensitive index for the domain purity in the GaAs/Si films.