In　this　paper,　an　on-line　monitoring　apparatus　of　fouling　thermal　resistance　is　introduced,　including　its　structure　and　perfoumance.　Based　on　the　fundamentals　of　heat　transfer,　heat　transfer　performance　was　tested　using　smooth　tube.　As　a　result,　the　heat　balance　deviations　of　two　sides＇　water　were　no　more　than　10　percent.　Experimental　turbulent　flow　heat　transfer　Nusselt　numbers　agreed　with　the　predicted　values　within　10%.　Accordingly,　the　system　is　reliable　to　be　used　to　monitor　the　fouling　process　of　heat　transfer　surface.