Bilayer graphene, or two layers of graphene stacked together in Bernal stacking, is a unique two-dimensional electron system with hyperbolic bands and a band gap tunable by the application of an electric field through the two layers. In this talk, I will describe our experiments in understanding the mechanisms of carrier transport in a gapped bilayer graphene. I will also show accurate measurements of the effective mass of bilayer graphene, from which we determine its band structure. Our results show that electron-electron interaction renormalizes the bands of bilayer and suppresses the effective mass m*. The manifestation is qualitatively different from that in conventional 2D systems.