Coated microbubbles used as ultrasound contrast agents show complicated nonlinear oscillation if insonified. The Marmottant model assumes the experimental surface tension that changes with the bubble radius. Though this model has been widely used in recent years, its physical background is still unclear. In this work, we focused on the molecular level state of the membrane of a coated microbubbles and performed molecular dynamics (MD) simulations of a water-membrane-gas interfacial system. For efficient calculations, the Coarse-Grained (CG) model was used. In this model, the group of atoms is replaced by one effective particle. The calculation efficiency is greatly improved, because the computational cost for interaction calculations is proportional to the power of the number of particles. We obtained stable structures of amphiphilic membrane molecules at different membrane surface densities that are related to the bubble size. The results of membrane thicknesses show qualitative change of the membrane state, and imply the possibility of the phase transition.