Abstract: In this study the effect of MWCNT nanofluid, as absorbing medium, on the efficiency of a flat-plate solar collector was investigated experimentally. The weight fraction of CNTs was 0.2% and 0.4%. The effect of Triton X-100 as a surfactant on the stability of nanofluid was studied. The tests were performed in different mass flow rates of nanofluid from 0.0167 to 0.05kg/s. ASHRAE standard was used to perform the tests. Results show that by increasing the weight fraction from 0.2% to 0.4%, there is a substantial increase in the efficiency. Also, using the surfactant causes an increase in the efficiency. [Copyright &y& Elsevier]