Natural Rubber (NR) modified with Phosphorylated Cashew Nut Shell Liquid prepolymer (PCNSL) or with an aromatic oil shows an increased pseudoplasticity index and a decreased consistency index. The apparent melt viscosity of these systems decreases with increased temperature and shear rate and the activation energy for melt flow also decreases with rate of shear. Further, the reduction in die-swell ratio during extrusion of the PCNSL modified NR indicates a reduction in melt elasticity compared to the unmodified sample. Good compatibility between NR and PCNSL is indicated by the decrease of the principal normal stress difference of NR in presence of PCNSL.