Wear tests between phosphorous bronze and steel, which are widely used as components of worm gear, were carried out with the Soda four-ball testing machine, consisting of a rotating steel ball and three fixed phosphorous bronze balls. The effect of lubricating oil additives on wear were investigated and the following results were obtained.(1) In the case of refined straight oil, wear was reduced with the increase of viscosity of the oil.(2) Organic acids, fatty oils, esters, alcohols and naphthenic soaps were used as additives in these experiments. Most of them exhibited the capacity of reducing wear; above all, a long chain fatty acid such as stearic acid was most effective.(3) Sulfur compounds such as sulfurized fatty oil or dibenzyl-disulfide and chlorinated paraffin remarkably promoted wear.(4) Phosphorous compounds such as phosphites and phosphates were highly effective in the presence of sulfur or chlorine compounds. Dialkyl dithiophosphates containing phosphorous in the molecular structure, and generally used as antioxidants, also promoted wear.(5) Some kinds of commercial EP additives for multipurpose gear oil promoted, while other kinds of them reduced wear.(6) The wear reducing action of additives such as fatty oil and phosporous compounds is observed also in the rubbing of steel against various copper alloys and in that of soft steel against phosphorous bronze.(7) It was recognized that there was a close relation between the roughness of rubbing surface and the amount wear. The roughness of rubbed surface was small for the additive of reducing wear, while it was increased for the additive of promoting wear.