In the present work, three-dimensional numerical study was carried out to perform the single- and multi-objective optimization process of delta winglet pair (DWs) mounted on a rectangular duct with a hydraulic diameter of <i>D<sub>h</sub></i> = 0.0535 m. For the purpose of presenting the consistency of numerical model with experimental data, experimental work was also performed for DWs with an angle of attack of θ = 40° ranging from Re = 5000-20,000. The response surface method (RSM) with central composite design (CCD) was performed to construct numerical design parameters as well as to obtain responses for Nu,<i> f</i>, and thermal enhancement factor (TEF). On this basis, the DWs' angle of attack (θ), the ratio of the transverse distance between the trailing edges of DWs to the duct width (<i>L<sub>VG</sub>/W</i>), and the Reynolds number (Re) were considered to be design parameters. The selected single-objectives are maximizing Nu, minimizing <i>f</i>, and maximizing TEF. Moreover, the chosen multi-objectives are maximizing Nu while minimizing <i>f</i>; maximizing Nu and TEF while minimizing <i>f</i>. The findings indicate that the angle of attack (θ) and Re have the greatest influence on all responses compared to the <i>L<sub>VG</sub>/W</i> ratio. For single-objectives, increase in Nu, decrease in <i>f</i>, and increase in TEF were attained on the order of 45.67%, 14.89%, and 3.98%, respectively, in comparison to the reference case. Furthermore, maximizing Nu while minimizing <i>f</i> was selected as a multi-objective, increase in Nu and decrease in <i>f</i> were calculated to be 36.38% and 12.76%, respectively, compared with the reference case.