Two compositions of immiscible systems - Cu50Cr50 and Fe86.5Cu13.5 - were selected for study of phase composition and crystalline structure transformation kinetics during ball milling of powder mixtures. Samples were milled in planetary ball mill in hermetic vials using two different rotation velocities of vials and two ball's diameters. The curves of the phases content vs. milling time were sigma -shaped for Cu50Cr50 composition. Complete transformation of alloy into b.c.c, phase was not observed for this composition, a steady-state ratio between phases amounts was reached after certain milling time. On the contrary, for Fe86.5Cu13.5 composition a complete transformation of f.c.c. phase into b.c.c. at mechanical alloying was observed. The curve of b.c.c. phase content vs, milling time includes two periods corresponded to the different transformation rates. Particular attention has been given to the initial part of phase transformation curves. The discussion of the results is based on the model of thermodynamic driving forces of mechanical alloying process.