Fouling　of　heat　transfer　surfaces　during　subcooled　flow　boiling　is　a　frequent　engineering　problem　in　process　industries.　It　has　been　generally　observed　that　the　deposits　in　such　industrial　systems　consist　mainly　of　calcium　carbonate　(CaCO3),　which　has　inverse　solubility　characteristics.　This　investigation　focused　on　the　mechanism　to　control　deposition　and　the　morphology　of　crystalline　deposits.　A　series　of　experiments　were　carried　out　at　different　surface　and　bulk　temperatures,　fluid　velocities　and　salt　ion　concentrations.　It　is　shown　that　the　deposition　rate　is　controlled　by　different　mechanism　in　the　range　of　experimental　parameters,　depending　on　salt　ion　concentration.　At　higher　ion　concentration,　the　fouling　rate　increases　linearly　with　surface　temperature　and　the　effect　of　flow　velocity　on　deposition　rate　is　quite　strong,　suggesting　that　mass　diffusion　controls　the　fouling　process.　On　the　contrary,　at　lower　ion　concentration,　the　fouling　rate　increases　exponentially　with　surface　temperature　and　is　independent　of　the　velocity,　illustrating　that　surface　reaction　controls　the　fouling　process.　By　analysis　of　the　morphology　of　scale,　two　types　of　crystal　(calcite　and　aragonite)　are　formed.　The　lower　the　temperature　and　ion　concentration,　the　longer　the　induction　period　and　the　higher　the　percentage　of　calcite　precipitated.