We　present　systematic　studies　of　the　transient　dynamics　of　GaAs　by　ultrafast　time-resolved　reflectivity.　In　photoexcited　non-equilibrium　states,　we　found　a　sign　reverse　in　reflectivity　change　Delta　R/R,　from　positive　around　room　temperature　to　negative　at　cryogenic　temperatures.　The　former　corresponds　to　a　free　carrier　metallic　state,　while　the　latter　is　attributed　to　an　exciton　insulating　state,　in　which　the　transient　electronic　properties　is　mostly　dominated　by　excitons,　resulting　in　a　transient　metal-insulator　transition　(MIT).　Two　transition　temperatures　(T-1　and　T-2)　are　well　identified　by　analyzing　the　intensity　change　of　the　transient　reflectivity.　We　found　that　photoexcited　MIT　starts　emerging　at　T-1　as　high　as　similar　to　230　K,　in　terms　of　a　dip　feature　at　0.4　ps,　and　becomes　stabilized　below　T-2　that　is　up　to　similar　to　180　K,　associated　with　a　negative　constant　after　40　ps.　Our　results　address　a　phase　diagram　that　provides　a　framework　for　the　inducing　of　MIT　through　temperature　and　photoexcitation,　and　may　shed　light　on　the　understanding　of　light-semiconductor　interaction　and　exciton　physics.