Recently, the application of nanoparticles has been extended in areas such as medicine, drug, agriculture, industry and environment. In medicine, different types of these nanoparticles have been applied in therapy of cancers, wounds, infections, and also drug delivery. Among these, graphene is a new 2-dimensional material with special properties like high surface area, high electrical and thermal conductivity, mechanical, biocompatibility and low cost in large scale production. For example, today, graphene nanoparticles are used for drug delivery, photothermal cancer therapy, biosensors, biocompatible scaffolds, bioimaging, and anti-microbial components. Graphene oxide was synthesized from natural graphite powder according to Hammer’ s modified method. Then, bacterial toxicity of synthesized graphene was evaluated in 1, 10 and 100 µ g/ml of E. coli and staphylococcus aureus. The AFM analysis showed that thickness of graphene sheets was 0. 5 nm and after reduction reached to 1. 4 nm. Our result showed that, antibacterial properties of graphene and reduced graphene were highly concentration depended and in 10 and 100 µ g/ml significantly decreased bacterial growth rate.