This report treats the effect of cutting fluids on surface roughness in key-broaching a carbon steel. There are two important factors to affect the surface roughness, that is, chip formation and built-up-edge or welding. Moreover, the surface roughness is a function of the cutting distance, and the latter cause is the more controlling for such the transient phenomenon, which is more emphasized by the transient characteristic of lubrication in wet broaching. Here is derived a coeffi-cient of roughness increase, which can be an index for the evaluation or applicability of the cutting fluids. Another finding is that the effect of a cutting fluid from the aforementioned point of view is closely associated with the ratio [load/wear width] in the four-ball test.