Graphene band structure can be modulated when dopant atoms are introduced into graphene sheets. As a result, there is flexibility in design and optimisation of electronic devices. In this study, the effects of atomic doping to graphene band structure were investigated by using boron and oxygen as dopant atoms. Different dopant concentrations and dopant locations in graphene sub lattices were studied by using a 4x4 graphene sub lattice which consists of 32 carbon atoms. Results show that both dopants cause opening of energy band gap of mono layer graphene. The highest energy band gap (Eg) value for graphene doped with boron is 0.52 eV and the highest Eg value for graphene doped with oxygen is 1.67 eV, in which both results are obtained for highest dopant concentration and farthest dopant's distance in a graphene sheet. This shows that higher dopant concentration and farther dopant's location in a graphene sheet lead to higher energy band gap.