The objective of this work is to analyze the performance of a milling and grinding process for soybean expeller, which is a by-product of the oil extraction by means of the extruding-pressing method, for its later use in obtaining protein products. Milling is done with three different mills: balls (impact), rollers (shear and compression) and blades (shear), under different operating conditions. The analysis of the grinding operation is done using the ASTM sieve series Nº 5, 6, 10, 12, 25, 30 and pan. The obtained products are classified in fine, medium and large, thus analyzing the yields of each fraction in each mill. Being the interest’s fraction the corresponding to medium sized particles, it is concluded that the roller and blades mills are the most adequate once since yields greater than 55% are achieved when operated with a single pass. Also, the specific energies developed for the milling were calculated with the Bond’s Law for comparison purposes.