A composition-dependent microphase separation of segmented poly (urethane urea)s (PUUs), based on a mixture of two hydrophobic (polycaprolactone) and hydrophilic (polyethylene glycol) polyols, is investigated. Synthesis of PUUs was carried out through the reaction of in situ generated AB-type macromonomers, prepared from the reaction of NCO-terminated urethane prepolymers, with benzoic acid in dimethyl sulfoxide as solvent/reagent at 40-80 °C. The segmented PUUs were characterized by different methods including FTIR and NMR spectroscopies, gel permeation chromatography, differential scanning calorimetry and dynamic mechanical analysis. Microphase separation in the synthesized PUUs was monitored using atomic force microscopy (AFM) to find a better insight into structure-property relationship of PUUs consisting of mixed polyols. Thermal analysis of the polymers revealed that by introducing poly (ethylene glycol) (PEG) in PUU backbone, a well-defined glass transition was obtained. The results of AFM showed that PCL-based poly (urethane urea) has a morphology in which hard segment domains were homogeneously distributed in the soft segment matrix. In the samples based on PCL/PEG, the hard segment domains aggregates were connected to each other and were inhomogeneously distributed in the matrix. Comparison of the overall data revealed that the differences in soft segment compositions had a marked effect on the molecular structure and the mechanical properties of PUUs.