Prosopis alba (algarrobo) flours are traditional food resources from Argentina. The aim of this work was to determine the effect of particle sizes of Prosopis flour on its chemical composition and functional properties. Flours were obtained by mechanical methods (F1 to F4, >840, 840 to 500, 500 to 149 and <149 µm, respectively). F1 and F2 contain 40% mesocarp while F3 and F4 show 80 and 90%, respectively. Mesocarp reserve parenchyma is rich in free and bound phenolic compounds, carbohydrates and fibers of the vascular system, CaCO3 crystals, and minerals (Fe, Ca, Mg, K). Apigenin C-glycosides and phenylpropanoids acids were identified in all fractions whereas F4 showed the highest level. All fractions showed functional properties. F3 and F4 showed the highest activity as H2O2 and HO• scavenger. F1 and F2 showed the highest activity as reductor agents and lipoperoxidation inhibitors. Flours with different granulometry may be suitable as functional ingredient or nutraceutical whereas flours with a smaller particle size (F3 and F4) are more interesting as functional ingredients because of their high content of bound phenolic compounds and particle homogeneity and could be used either alone or together.