A laboratory scale Szegvari type vertical mechanical alloying (MA)/mechanical milling (MM) attritor for research purposes was designed and constructed at Gazi University. By using this attritor, optimum processing parameters such as milling time, milling speed (rev/min), diameter and the amount of milling balls, milling atmosphere were determined when MM'ing of gas atomized 17-4PH stainless steel powders were carried out. After the milling, powder size analyses and microscopic studies (optical and SEM) were utilized for characterization of the as-received and milled powders. It was observed from the results of the present investigation that for 17-4PH stainless steel (SS) powders, optimum properties were obtained when mechanical milling at Gazi University attritor was carried out at 500 rev/min in Ar gas by using 250 g of 10 mm diameter hardened chromium steel balls. It was also observed that MM time has a profound influence on the characteristics (such as particle size, shape and distribution) of MM'ed 17-4 PH gas atomized SS powders depending on the two competing mechanisms, namely, the rate of welding and the rate of fracturing. © 2005 Elsevier Ltd. All rights reserved.