Mixing of different polymers with each other (polymer blending) is a new method to produce materials with superior properties in comparison with their initial components. In recent years, new types of polymer/polymer composites has been emerged and have attracted much attention due to their superior properties such as low density, proper process ability, low cost and no need to the dispersion of fiber in to the matrix. One of the polymers pair which has high potential to produce these kinds of composites is a combination of polyethylene (PE) and polyamide6 (PA6). In this polymers pair, PE acts as a matrix and PA6 is reinforcement and their microfibrillars reinforcement will be formed in situ while processing. Since the final properties of the composite will be highly influenced by the morphology of reinforcement component, in this thesis, morphology development and its effects on the rheological behavior of final composite is studied in detail. In this regard, 3 types of PA6 and 2 types of PE with different viscosity was used to prepare the microfibrillar composite with a constant weight ratio of 10% and a constant draw ratio of 7. Next, by using SEM, RMS, MFI and contact angle tests, the properties of new composites have been investigated. According to the SEM results, the droplet-matrix morphology was formed before the extensional flow field. These droplets were converted to the fibers with different diameter in extensional flow. According to the rheological properties of raw material, it can be said that final morphology depends on viscosity ratio, elasticity ratio and coalescence phenomenon. So that with an increase in viscosity of disperse phase in a constant matrix, middle ratio of viscosity and elasticity cause to stretching of the droplets in the field and forming fibers with 2.75?m in diameter. However, in higher or lower viscosity ratio average diameter of the fibers is 6.86 and 4.55?m respectively. This result is related to the simultaneous effect of elasticity and coalescence.In the case of matrix viscosity, it was observed that a decrease in matrix viscosity in a constant viscosity of disperse phase, the average diameter of fiber is increased; mainly due to high possibility of coalescence phenomenon in a matrix with lowers viscosity. Study of final composite rheology indicates an increase in viscosity and loss modulus decrease in damping factor with decrease of fiber diameter. This results show that an entanglement network of fiber with high length to diameter ratio is formed. Another important aspect is the viscosity and modulus of the fibers. It can be concluded that with higher viscosity and modulus of fibers, higher viscosity and modulus in final composite is achievable. Also lower surface interfacial tension components cause to higher adhesion of two phases and lead to higher the viscosity of composite. Hence, the morphology and rheology of final composite is affected by the above mentioned parameters. Key words: Rheology, Morphology, Microfibrillar composite, Polyethylene, Polyamide6, Viscosity, Elasticity.