In current seismic design code of each country in the world, the mainstream of earthquake input for calculating the building is to use X and Y directions conveniently. The X and Y directions are the main axis of the building, and the strong-column weak-beam damage mechanism has been considered only followed to these main axis. However the building response in the oblique direction is higher than in the main axis, and the frequency in the oblique direction of the building is higher than the X or Y direction as well. Moreover the column usually connects with two beams in the orthogonal directions and the strength ratio of the column to beams in the oblique direction is less than in X or Y direction. Therefore it could be the major reason to cause the column-plastic-hinge damage mechanism during the earthquake. In this paper, it is discussed how the seismic input in the oblique direction influence on the strong-column weak-beam damage mechanism for RC frame through the perspectives of the dimensional mechanical analysis, cyclic loading experiment and the finite element analysis for the dimensional column-beam joint, and then proposed that considering the seismic input in the oblique direction is necessary in the future structural design in order to reach the actual strong-column weak-beam mechanism.