The kinematic-geometric lubrication theory, which combines the results from the elastohydrodynamic lubrication theory and the meshing theory for gearing, is put forward for a line-conjugate gear drive to investigate its global intrinsic lubricating characteristics to guide its geometric design. The investigation concerning the global lubricating characteristics of the gear drive can be well utilized to seek the lubricating weak zone in the whole meshing zone and to determine the reasonable operating parameters for it. The relationship between the lubricating angle and sliding angle is made clear, and the lubricating angle is recommended to determine the lubricating weak point. The kinematic-geometric coefficient of oil film thickness is proposed to evaluate preliminarily the lubricating characteristics of the gear drive. This theory is applied to the arc-toothed cylindrical worm drive, and it is discovered that there is one path of lubricating weak points on the tooth surfaces. The path is from the worm gear addendum to dedendum and is almost impossible to be eliminated via analysis and research. The path is roughly located in the middle of the worm gear tooth surface, and a lubricating weak zone exists around this path whose area roughly accounts for one-third of the whole meshing zone.