The use of nanofluids in heat exchangers is known as a method that increases the amount of heat transfer and the thermal efficiency of the heat exchangers. In this study, the effect of using TiO2/water (Titanium dioxide-base fluid) nanofluid on the thermal performance in a counter current parallel flow plate heat exchanger was investigated. Three different fluids were used as hot fluids in the experiment, and these are water, TiO2/water nanofluids with 0.25% and 0.5% volumetric mixing ratios. In order to understand the effect of nanofluid on thermal performance, first water, which is the base fluid, was used in the experiments, and then nanofluids were used. Four different values were selected, with a flow rate of 4.5,6,7 lt/min of water used as cold fluid. For each flow rate, separate experiments were carried out at constant temperature values where the hot fluid inlet temperature was 40 ̊C, 45 ̊C, 50 ̊C, 55 ̊C and 60 ̊C and heat transfer, effectiveness and improvement rates were examined. According to the data obtained from the experiment, it has been determined that the use of TiO2/water nanofluid instead of water as the hot fluid in the plate heat exchanger improves the thermal efficiency. The greatest increase in heat transfer was achieved in the experiment using TiO2/water nanofluid, where the cold fluid flow rate was 7 lt/min, the hot fluid inlet temperature was 60 ̊C and the volumetric mixing ratio was 0.5%. This value is calculated as 14508 W. According to the results obtained, the highest improvement rate was obtained as 14.3% in the study where TiO2/water nanofluid with a volumetric mixing ratio of 0.5% was used as the test fluid and the cold fluid flow rate was 4 lt/min. In addition, as the flow rate increased, the heat transfer amount increased and the improvement rate decreased.