Nowadays, in order to accommodate the growth in demand for computing resources, adding more power and cooling capacity to existing data centers or build new data centers is required. However, both of these options are extremely expensive. Therefore, improving energy efficiency has become a paramount necessity for data centers. The right architecture and topology design will be able to produce effective use of energy and increase performance. Taking into account the servers and network without the cooling aspect in data center energy efficiency, and the increasing trend of network simulator in data center network research, this paper aims to answer the emerging question by comparing two most common network topologies (Fat-Tree and B-Cube) in data center using Network Simulator 3 (NS3) tool. Despite having difference in nature, those two topologies have an equalized form to be compared. In this research, the focus of energy efficiency study is to determine whether the difference of nature in Fat-Tree and B-Cube topology will affect the energy consumption in the data center using an assumption of energy consumption for a certain network device. The result, despite the small amount, B-Cube is proven to be 0.003141% better than Fat-Tree in term of energy use.