An　experimental　study　was　carried　out　by　a　dynamic　fouling　monitor　system　to　investigate　the　fouling　process　in　convective　heat　transfer　by　ultrasonic　treatment.　During　the　experiment,　the　cooling　water　in　heat　exchanger　was　used　as　working　fluid　with　the　inlet　temperature　of　22.5　degrees　C　and　44　degrees　C　and　the　initial　hardness　of　300　mg/L　and　500　mg/L,　respectively.　For　all　cases　the　inlet　temperature　of　hot　water　was　kept　at　70　degrees　C,　and　the　flowrates　of　cooling　water　and　hot　water　were　set　at　0.77　m(3)/h　and　0.81　m(3)/h,　respectively.　In　this　experimental　setup,　a　double-tube　heat　exchanger　was　served　as　a　test　section　of　heat　transfer,　in　which　hot　water　flows　inside　the　inner　copper　tube　and　cooling　water　flows　in　the　annular　gap　between　the　two　tubes,　thus　forming　a　counterflow　situation.　Further,　an　ultrasonic　device　was　installed　for　water　treatment　with　a　frequency　of　20.7　kHz　and　power　ranging　from　0　to　75　W.　The　results　showed　that　the　fouling　resistance　for　hard　water　increased　evidently　with　increasing　of　water　temperature　and　hardness.　With　the　ultrasonic　treatment,　the　fouling　resistance　decreased　remarkably　compared　with　the　untreated　case,　and　the　asymptotic　fouling　resistance　decreased　monotonously　with　increasing　of　the　ultrasonic　power.　Subsequently,　the　crystal　morphology　of　calcium　carbonate　was　observed　in　microscopic　view　and　the　transform　of　crystal　from　the　vaterite　to　aragonite　and　calcite　was　analyzed　based　on　the　theory　of　the　formation　energy.　It　was　confirmed　that　the　ultrasonic　treatment　may　have　significant　effect　on　the　crystal　shape,　and　more　aragonites　appeared　with　increasing　powers　of　ultrasound.