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This study deals with experimental investigation and constitutive modeling of the mechanical behavior, damage, and fracture of polymer/clay hybrids. The attention focuses on unexpected features of their viscoplastic responses, damage accumulation, and crack propagation. 2 Preparation of nanocomposites Nanocomposites were manufactured by using isotactic polypropylene Moplen HP 400R (Albis Plastic Scandinavia AB, Sweden) as a matrix, organically modified montmorillonite nanoclay Delitte 67G (Laviosa Chimica Mineraria S.p.A., Italy) as a filler, and maleic anhydride grafted polypropylene Eastman G 3015 (Eastman Chemical Company, USA) as a compatibilizer. Hybrids were prepared by using a two-step melt mixing process: (i) masterbatches with various clay/compatibilizer proportions were extruded in a twin-screw extruder Brabender PL2000, and (ii) neat polypropylene and the masterbatches were melt mixed at various proportions (corresponding to various concentrations of nanoclay) at molding of dumbbell specimens in injection-molding machine Arburg 320C. Dispersion of clay was examined by means of rheological tests (small-amplitude shear oscillations). The graphs of storage and loss moduli demonstrated strong low-frequency plateaus conventionally associated with good exfoliation of nanofiller. 3 Tensile tests The effects of clay content and clay/compatibilizer proportion on the mechanical response of nanocomposites was analyzed in uniaxial tensile tests. Experiments were conducted by means of universal testing machine Instron-5568 equipped with a thermal chamber at various strain rates (ranging from˙ = 2 • 10 −4 to 10 −1 s −1) and various temperatures T (in the interval from room temperature to 100 • C). Observation demonstrate noticeable improvement of mechanical properties (Young's modulus and yield stress) of polypropylene induced by its reinforcement. As unexpected results, it is worth mentioning (i) a rather low critical concentration of nanoclay (about 1-2 wt.%), (ii) weak influence of clay/compatibilizer ratio on the response of hybrids, and (iii) enhancement of reinforcement at elevated temperatures