It is demonstrated that, using a suitable GaAs1-xPx buffer layer, stress free GaAs can be grown on Si (100) substrates. In fact the lattice mismatch between GaAs and GaAs1-xPx will introduce a (100) biaxial compressive stress component in the GaAs top layer. The magnitude of this lattice mismatch induced stress can be monitored from 1 to 7 Kbar by varying the P content of the buffer from × = 0.02 to × = 0.15. Results on GaAs/ GaAs1-xPx/ Si (100) ( but also on strained GaAs grown on GaAs1-xPx/GaAs(100)) are discussed in the light of photoluminescence and X-ray diffraction experiments. It is shown that, by adjusting the P composition in such buffer, the lattice mismatch induced stress can exactly compensate the thermoelastic stress, leading thus to an almost stress free GaAs at room (300K) or at low (2K) temperature.