Based on industry demands, computer numerical control (CNC) milling is introduced as a new technology to cut spiral bevel gears as well as the conventional gear manufacturing approaches. End milling and flank milling are two modes of CNC milling. Compared to end milling, flank milling has been highlighted in quality enhancement, manufacturing time and cost reduction. Theoretically, the result of flank milling a developable ruled surface does not contain geometric deviations. Consequently, flank milling has been widely used to machine the parts with ruled surface design. However, there is no ruled surface design available for the tooth surface of spiral bevel gears (except Format-cut tooth surface, which is part of a cone), although many tooth surface models used in conventional approaches are close to the ruled surface. Moreover, there is no literature on five-axis flank milling spiral bevel gears. To address these two problems, a new ruled tooth surface design is proposed to be machined with five-axis flank milling. Furthermore, the geometric meshing theory is introduced to obtain the closed-form representation of the simulate machined tooth surface. The result of the proposed design is compared with the result of conventional model. The comparison shows that the proposed design reduces the geometric deviations effectively. Moreover, a pinion is also generated to mesh with the simulate machined gear. The result of tooth contact analysis (TCA) shows that the proposed method is effective.