A three-dimensional study using the standard k-ε turbulence model to simulate airflow in and around a natural draft dry cooling tower (NDDCT) has been conducted using a general-purpose CFD code. This investigation considered the location and the porosity of windbreak walls' structure on the NDDCT thermal performance. In addition, the effect of the windbreak walls on the thermal performance of two NDDCTs with different capacities has been investigated. Two parameters have been used to show the effect of the windbreak walls on the NDDCT thermal performance. At the reference heat exchanger temperature, the thermal effectiveness parameter has been employed. At the reference rejected heat from the NDDCT, the change in the cooling tower approach parameter has been employed. The results in this paper show an improvement in the NDDCT thermal performance due to the introduction of windbreak walls. Moreover, optimizing the location of the windbreak walls has been shown to have a more significant effect on the NDDCT thermal performance than the porosity of the walls. In addition, the effect of the windbreak walls on the thermal performance is similar for the two NDDCT with different capacities.