Spur gears with profile contact ratios ranging from 1.62 to 2.15 were mirrorlike finished to an accuracy of JIS 0 class using a CNC gear grinder designed and made by the authors. Elastic deformations of teeth and the resulting rotational errors of test gears were calculated, and then the amount and the starting point of profile modifications were decided by considering both the total elastic deformation and the pitch errors of the gears. In the case of non-modified gears, running noise and vibration could be decreased when the profile contact ratio was made greater than two. They could be decreased further when a proper modification was given to the gears. Experimental results suggested that the proper amount of modification is a little greater than the elastic deformation at the tooth tip when the proposed load (full load) is assumed to be applied at a point, where the modification start, which is a little higher than the pitch point.