Aim: To assess the characterization, antimicrobial activity of green synthesised magnesium oxide nanoparticles using Psydium guajava leaves. Materials And Methods: Psydium guajava leaves were dried and ground, of which 2 g of the powder was used for the study. the powder was then dissolved in 100ml of distilled water. 1 mg of magnesium chloride salt was dissolved in 20 ml of distilled water. Both the solutions were mixed and subjected to centrifuge. After the preparation of the nanoparticle, it was subjected for further investigation like SEM analysis, EDAX, UV-VIs, cytotoxicity and anti microbial tests. The cytotoxicity was investigated using a brine shrimps bioassay. The number of surviving naupli cells were manually counted under dissection microscope. The antibacterial activity was examined against four bacterial strains- E.coli, E.fecalis,S.aureus,P.aurogenosa by agar well diffusion method .The inoculated plates were incubated overnight (for about 24 hours at 37 degree celsius) and the inhibition zone around the nanoparticle solution was calculated. The antimicrobial tests were carried against standard antibiotic and the results were tabulated. Results: SEM analysis showed a particle size of 70 nm and the UV -Visible spectroscopy showed a peak at 320nm confirming the presence of magnesium oxide nanoparticles that is prepared using Psydium guajava. Cytotoxicty tests revealed that 83% of the brine shrimp cells survived .The anti microbial tests revealed that the nanoparticle was effective against two micro-organisms -S.aureus and E.fecalis and their zone of inhibition was nearly equal to that of standard antibiotic against which the organisms were incubated. Conclusion: Psydium guajava synthesized magnesium nanoparticles were found to have good cytotoxicity and acceptable anti-microbial activity especially against two most commonly harbouring micro-organism(S.aureus and E.faecalis) .Hence they can be extended for use in soft tissue healing.