Different strategies have been proposed to improve the efficiency of the energy conversion systems. One of the outstanding technics for heat transfer increment is utilizing nanofluid as working fluid. This experimental study investigates the effect of using kaolin/deionized water nanofluid that is comprise various kinds of metal oxides in different ratios, on thermal behavior of counter flow concentric tube (CFCT) heat exchanger and parallel flow concentric tube (PFCT) heat exchanger. The prepared kaolin/deionized water nanofluid solution has 2% (wt/wt) nanoparticle concentration. Also, to prevent flocculation and to enhance stability of the prepared nanofluid, sodium dodecyl benzene sulfonate surface-active agent has been mixed into the solution. The experiments have been conducted in various conditions to investigate the influence of using nanofluid. Utilizing kaolin/deionized water nanofluid as working fluid caused maximum improvement of 37% and 12% in overall heat transfer coefficient in CFCT and PFCT heat exchangers respectively, in comparison with deionized water.