In this paper, the functions of grinding fluids are discussed experimentally on ground surface roughness of carbon steel, cast iron and brass. Finished surfaces are observed with a surface roughness meter and a microscope, and grinding forces are measured, in dry grinding and wet grinding with each fluid tested repeated alternately under same conditions. And then the performance of the fluids is evaluated on comparing wet grinding results with dry ones, and the functions of the grinding fluids are discussed on the basis of those experimental results. As a result, it is found that, in the grinding of steel and cast iron the main functions of the grinding fluids on the surface finish are following two; (1) to prevent welding chips on grains, (2) to lubricate between grains and ground surface, and the preventive function against the welding of chips influences remarkably more than the lubricating function on the surface roughness, and in the grinding of brass, rough ground surface tends to be produced because of the phenomenon that the chips weld on the work surface and the grains, and hence the surface finish is remarkably improved due to be prevented welding the chips by the use of the grinding fluids.