The synthesis of magnesium oxide nanoparticles (MgO-NPs) using green synthesis has increased. In this study, green synthesis of MgO-NPs has been carried out by using Annona muricata. Annona muricata, popularly known as “graviola” or soursop, is a typical plant in countries with a tropical climate. The MgO-NPs were synthesised by mixing 30 mL of Annona muricata extract with 5 mM magnesium nitrate solution. The synthesised MgO-NPs were characterised by using Thermogravimetric Analysis (TGA) to identify the calcination temperature to form MgO-NPs, followed by a Fourier Transform Infrared (FTIR) analysis, X-ray Diffraction (XRD) analysis and Scanning Electron Microscopy (SEM). From TGA, temperatures at 700 and 900 have been chosen. For FTIR characterisation, the 450-600 cm-1 peaks were assigned to MgO stretching vibrations. Under SEM, the morphology of synthesised MgO-NPs seems to be in irregular shape and the aggregation of particles were observed.