Abstract: A hinge assembly of the present invention includes two parallel axle members, two gears and a transmission member. Each of the axle members has an axis and is rotatable about the axis. The gears are spaced apart from, each other, are mounted respectively and co-rotatably to the axle members, and are disposed on a plane that is perpendicular to the axis of the axle members. The transmission member is rotatable about a transmission axis that is perpendicular to and coplanar with the axes or the axle members, and meshes with the gears such that when one of the axle members rotates, the other one of the axle members is driven to rotate via the gears and the transmission member.