The paper is focused on the analyses of the mechanical and physical properties of the hybrid composite systems based on the polypropylene matrix with implemented reinforcing cellulose fibres. The polymer phase of this conventional composites is furthermore modified by physical blowing agents, which are utilized for the formation of light microcellular structures. Interactions of all the components of these hybrid composites are evaluated in terms of density change, impact resistance, bending and tensile properties. Complying with the assumption closed pores generally decreased mechanical properties of the multiphase composite systems. The reduce of mechanical properties induced by presence of the microcellular structure was more significant with increasing level of filling by reinforcing particles.