Two-phase　flow　patterns　on　gravity　heat　pipe　have　an　important　influence　on　working　characteristics.　The　paper　uses　for　reference　two-phase　flow　patterns　transformation　in　avertical　pipe　experimental　research　result　and　observation　conclusion　of　approximately　saturable　freely-falling　film　flow　departure　harbor　boiling,　through　simplified　analysis,　the　paper　is　given　that　flow　patterns　transformation　criterion　which　from　the　bubbly　flow　transition　the　slug　flow　in　gravity　heat　pipe　evaporator.　At　the　low　saturation　steam　pressure　and　the　high　undercooling,　this　flow　patterns　transformation　will　conduce　periodic　oscillation　two-phase　flow　instability,　this　is　spew　instability.　Therefore,　flow　patterns　transformation　criterion　is　spew　instability　regulation　criterion.　According　to　numerical　calculation　example,　it　is　pointed　out　that　effect　flow　patterns　transformation　of　important　factors.