We have performed real time measurements of intrinsic stresses during growth by reactive dc magnetron sputtering of aluminum nitride (AlN) thin films on silicon substrates in an UHV growth chamber. An experimental setup based on laser beam reflection is constructed such that substrate curvature as well as film thickness can be continuously monitored as growth proceeds. On Si(111) substrates, stress measurements were carried out during growth of both polycrystalline and epitaxial A1N films as a function of deposition pressure. This is the first such comparative study to our knowledge for the AlN/Si system. Our room temperature measurements on polycrystalline films corroborates previous post-growth measurements. Our high temperature measurements provide evidence of large intrinsic stresses and negligible stress relaxation during epitaxial growth of AlN on Si(111). We further compared stress behavior during both room temperature and high temperature growth of AlN films on Si(111) and Si(001) substrates. Our observations indicate while intrinsic stresses during room temperature growth can be compressive or tensile depending on plasma conditions, it is tensile during late stage growth at high temperatures.