The growing demand for energy-saving in industries has led to numerous researches to improve the heat exchangers thermal performance in a variety of methods. The pillow plate heat exchanger (PPHE) is one of the new types of plate heat exchangers that have specific functional properties compared to conventional plate heat exchangers. The objective of the present investigation is providing comprehensive information about effective geometrical parameters of PPHEs for optimal design. In the present study, the performance of PPHEs is studied by numerical simulation. The obtained results have shown that the geometrical parameters of the PPHEs have a significant effect on thermal performance. The considered geometrical parameters include the height of pillow plate channels, and the longitudinal and transversal distance of welding spots. The heat exchanger thermal performance increases by expanding the height of the pillow channel mostly at low Reynolds number. After increasing longitudinal and transversal distance of welding spots the heat transfer and friction factor decreases in both cases. However, the effect of friction factor decrement on heat exchanger performance is more than decreasing in heat transfer. Thereby the PPHE with longer longitudinal or transversal distance of welding spot is efficient. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.