Two alternative orientations of a GaAs layer in respect to a vicinal (0 0 1) substrate, misoriented toward [1 1 0], are possible for most of the miscut angles; type A orientation having the GaAs [1 1 0] parallel to the [1 1 0] Si misorientation direction and type B with the GaAs [1 (1) over bar 0] parallel to the [1 1 0] Si direction. The dependence of the GaAs surface roughness and of the tilting between the GaAs and Si (0 0 1) planes on the GaAs/Si orientation and the miscut angle of the vicinal substrate, in the range of 0 9, has been investigated. The GaAs/Si surface roughness was characteristic for preferential growth along [1 (1) over bar 0]. Thin GaAs films of approximately 2 mum thickness exhibited an almost constant rms roughness of 6.5-7.0 nm for the type A orientation, while the rms roughness varied in the range of 4.2-60 nm for the type B orientation, depending on the value of the miscut angle. The tilting angle between the GaAs and Si (0 0 1) 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 B samples, in the range of miscut angles of 1.-7.5 degrees. The results suggest that the GaAs/Si lattice tilting may be a sensitive index for the domain purity in the GaAs/Si films.