The influence of sodium oleate additives on processing and morphology of polypropylene - co-polyamide (PP/CPA) mixture melts has been investigated. It is shown that sodium oleate is located in the interphase of the components and acts at small additives (up to 3 wt%) as an interfacial tension agent, improving miscibility of polymers, and increasing the kinetic stability of mixtures and the degree of PP dispersion in the co-polyamide matrix. The plasticizing effect on mixture melt was observed at 7 wt% sodium oleate as a reduction of mixture viscosity. The ultrathin PP fibers (microfibers) strictly oriented in the direction of extrusion are the main type of structure produced during extrusion of a PP/CPA mixture. Addition of sodium oleate changed the PP fiber-formation in the CPA matrix: the total number of fibers increased, their diameter and diameter distribution decreased.