Experiments　of　unstable　vapor　liquid　two　phase　flow　phenomena　during　flow　boiling　with　HFE-7100　in　two　parallel　rectangular　multi-microchannels　with　aspect　ratios　of　0.2364　and　0.3182　were　conducted　under　a　wide　range　of　test　conditions:　the　mass　flux　from　380　to　3500　kg/m2s,　the　heat　flux　from　0　to　1080　kW/m2　and　the　compressive　volumes　of　0,　5,　10　and　20　ml,　respectively.　First,　characteristics　of　flow　boiling　heat　transfer　and　two-phase　pressure　drop　in　the　two　parallel　multi-microchannels　are　compared　with　each　other.　It　shows　that　the　maximum　dissipated　heat　flux　in　the　microchannels　with　the　aspect　ratio　of　0.3182　is　34.4%　larger　than　that　in　the　microchannels　with　the　aspect　ratio　of　0.2364　while　the　pressure　drop　in　the　microchannels　with　higher　aspect　ratio　is　up　to　80%　less　than　that　in　the　microchannels　with　lower　aspect　ratio　under　the　test　conditions.　Then,　experimental　results　of　unstable　vapor　liquid　two　phase　processes　in　the　two　parallel　multi-microchannels　are　compared　with　each　other.　It　shows　that　the　minimum　heat　flux　decreases　with　decreasing　the　aspect　ratio　at　the　same　compressive　volumes.　Finally,　the　physical　mechanisms　of　unstable　vapor　liquid　two　phase　phenomena　in　the　parallel　multi-microchannels　have　been　analyzed　according　to　a　simplified　mathematical　model　based　on　the　mass　and　momentum　conservation.　It　is　concluded　that　a　wide　flat　regime　in　the　hydrodynamic　curves　may　decrease　the　unstable　vapor　liquid　two　phase　flow　phenomena.