In this paper molten high density polyethylene (HDPE) was compounded with four kinds of high performance organic fibers: two types of aramid (KF29 and KF49), liquid crystalline polymer (LCP) and poly(vinyl alcohol) (VF), differing in their chemical structure and fiber lengths. From the SEM pictures, it is observed that shape and size of these organic fibers maintained almost the same even after cutting in pellets, following the mixing process. VF/HDPE and LCP/HDPE systems show generally lower rates of increase of both storage modulus and dynamic viscosity with fiber content than KF/HDPE composites. Comparison of these functions at the fixed fiber content has shown that the most effective parameter affecting the viscoelastic behaviour of organic fiber filled systems, seems to be their rigidity/flexibility in the molten state. The influence of fiber rigidity/flexibility becomes gradually lower with the increase of both strain amplitude and angular frequency. The parameters of the equations describing relationship between relative values of viscoelastic functions and fiber content were found to be largely dependent on fiber content. Such finding remarkably differs from behaviour of short inorganic fiber filled systems, where these variables maintained constant values.