High-efficiency plate heat exchangers are widely used in many industries. Nanofluids can be used as working fluids in heat exchangers to improve the heat transfer rate. In this study, the effects of using a fly ash nanofluid in a plate heat exchanger were analyzed experimentally and numerically. ANSYS FLUENT software was used to simulate heat transfer characteristics in a plate heat exchanger. Fly ash nanoparticles with average sizes of 14 nm were used for making the nanofluid. The prepared nanofluid had nanoparticles of content 2% (wt./wt.). The experiments were carried out in various working conditions with changes in fluid mass flow rate and temperature. The obtained experimental results showed that using the fly ash nanofluid enhanced the overall heat transfer coefficient between 6-20%. Moreover, theoretical and experimental results are in good agreement with each other.