The Moon is recognized as an important destination for space science and exploration. The National Aeronautics and Space Administration (NASA) has presented a new policy for space exploration that includes human expedition to the lunar surface. The other agencies that include the Japan Aerospace Exploration Agency (JAXA) have also planned to send spacecraft to the moon. To put the spacecraft into orbit around the moon, various methods are conceived. For example, one of them is that the special spacecraft, such as an orbital transfer vehicle (OTV), is used in each transfer segment. It is the very important thing in cost that OTV between Low Earth Orbit (LEO) and Lunar Low Orbit (LLO) is reused. To promote lunar exploration efficiently, reusable OTV between LEO and LLO is required. OTV has been studied at JAXA, which is based on further development of H-II Transfer Vehicle (HTV). The OTV system is roughly composed of the payload segment, the propulsion segment and the fuel cartridge. The payload segment is separated from OTV body on Lunar Transfer Orbit (LTO), and is transported to LLO. The propulsion segment and the empty fuel cartridge are returned to LEO through Earth Transfer Orbit. (ETO). This paper aims at improving the payload ratio of reusable OTV. In this research, we focus on optimization problems of return trajectories with small fuel consumption. Flight time in LTO should be short generally. When payload is empty, flight time isn't restricted. So, multi-impulse flight using electrical propulsion is used in ETO. Aero-assisted flight using aero-brake is also utilized near the Earth. In ETO, we discuss ballistic flight using chemical propulsion, multi-impulse flight using electrical propulsion and aero-assisted flight using aero-brake. Optimal return trajectories of OTV between the Earth and the Moon are proposed. This research can be also extended to the transfer vehicles between the Earth and the Lagrange point of Sun-Earth system. The L2 point in the Sun-Earth system may be utilized for a space port for the outer planets exploration in the future. In this paper, we show the concept of the OTV system and the result of the optimal return trajectories for OTV.