We present a transmission electron microscopy (TEM) study of the dependence of structure defects on the kind (Ga or As) and the growth temperature of the prelayer in GaAs-on-Si films grown by molecular beam epitaxy (MBE) on vicinal (100) Si substrates. Generally the prelayer formation conditions do not change the appearance and the density of threading dislocations that remain the main structural defect. However the structural characteristics for each cut-off angle appear to depend eventually on the [011] direction of GaAs epilayer relatively to the [011] misoriented Si substrate direction. Planar defects can be avoided only for midrange angles (e.g. 4.5 degrees) using As prelayer at 400 degrees C.