Hydration　mechanism　of　tabular　alumina　carbon　composites　reinforced　by　Al4C3　in　situ　reaction　with　Mg　and　Al　was　researched　in　water　steam　using　super　automatic　thermostatic　water　bath　from　25　degrees　C　to　85　degrees　C.　It　is　shown　that　hydration　mechanism　of　the　composites　is　chemical　reaction　control　at　44.3.-84　degrees　C　in　H2O(g).　The　hydration　was　controlled　by　diffusion　from　24.7　degrees　C　to　33　degrees　C　The　ratio　of　added　Mg/Al　influences　the　HMOR　of　the　composites.　The　mechanism　of　HMOR　of　the　composites　with　different　ratios　of　Mg/Al　can　be　discovered　by　means　of　SEM　analysis.　The　active　Mg/Al　powder　and　flake　graphite　inside　give　the　composites　outstanding　hot　strength　resulting　from　the　interlocking　structure　of　Al4C3　crystals　at　high　temperature.　Besides,　the　matrix　changes　into　the　Al4C3　with　high　refractoriness.　The　method　of　preventing　the　hydration　of　tabular　alumina　carbon　composites　reinforced　by　Al4C3　in　situ　reaction　was　immersed　in　the　wax　at　suitable　temperature　or　storing　them　below　33　degrees　C　in　a　dry　place　or　storing　them　with　paraffin-coating.