Abstract: The interplanetary trajectories to the high latitude of the sun via gravity assists of inner planets were analyzed to reduce the energy cost of the space mission. Based on the patched-conic theory, the energy contour maps, which were used to search for the appropriate launch opportunities of multiple spacecraft and celestial body encounters, were plotted by solving the Lambert′s problem. The ratio of orbit periods between the spacecraft and the planet, such as Venus or Earth, was designed as 1∶ 1 or 2∶ 3 to provide repetitious gravity assists. Results show that, if inner planet gravity assists are used, the inclination of the object orbit relative to the ecliptic reaches about 30°at the most following thrice Venus or Earth gravity-assist flybys, and the "Earth-Venus-Mars-Earth-Earth-Earth" flight path which demands lower launch energy and shorter flight time is the optimal trajectory. Launching energy can be much reduced by multiple gravity-assist maneuvers than by single planet maneuver.