The aim of this work was to investigate the effect of synthesized magnesium oxide nanoparticles of Jatropha tajonensis leaf extract on the growth and yield of cowpea (Vigna unguiculata (L.) Walp.). Materials and Methods. The preparation and planting of the cowpea seeds; The extraction of extract of Jatropha tajonensis leaves in aqueous solution. The synthesis of MgO nanoparticles from the extract, followed by characterization to confirm the formation — UV-VIS, FTIR, SEM-EDX and PXRD. The effects of MgONPs on cowpea (Vigna unguiculata (L.) Walp.) plants were surveyed under field conditions to assess its uses in improving growth and yield of cowpea. Results. The results showed that different doses of MgONPs applied to cowpea plant significantly affected all measured parameters of cowpea plantlets under the field condition in a positive way. The best results in growth, yield and the phonological parameters were cowpea plants treated with high MgONP applications (100 mg/L). It has been observed that different MgONPs applications have significant effects on vegetative growth and yield parameters of cowpea. A significant increase in the number of vegetative parameters was observed in the pots with different doses of nano-20, 40, 60, 80 and 100/MgONPs applications compared to the control. Different MgO (with or without NPs) treatments led to significant differences in shoot formation (P < 0.01). According to the effect of different doses of magnesium NPs applied to the cowpea, plant height varied between 18.88 ± 2.51 and 21.35 ± 3.25. The highest value in the height was obtained from nano-100 mg/L MgONPs application with 21.35 ± 3.25 and the lowest value was obtained from the salt 17.48 ± 3.83 mg/L MgONPs application. Conclusion. This study found that MgONPs greatly influenced the plantlets’ growth parameters and other measured traits; in addition. There was an indication that the efficiency of growth and yield of cowpea could be improved by increased application of MgO in the form of nanoparticles. Also, highlighted was the possibility of using MgONPs in increasing another crop yield to cater for the evergrowing world population.