This study presents a new approach to reinforce and toughen thermoplastic polymers by using a low molecular weight additive; calcium stearate (CaSt(2)). The method involves melt blending isotactic polypropylene (iPP) and CaSt(2) at a process temperature where the CaSt(2) reduces the process viscosity through its characteristic morphology. Once the mixture cools, CaSt(2) coalesces and solidifies to develop reinforcing domains. Differential scanning calorimeter experiments demonstrate a significant reduction in the percent crystallinity of iPP with the addition of CaSt(2). The changes in yield strength and modulus suggest a synergistic effect between the CaSt(2) domains and iPP. Addition of 10% CaSt(2) increases the fracture energy by almost four-fold with a simultaneous increase in the elastic modulus. The melt flow rate of the composite is dramatically increased with CaSt(2) incorporation. Reduction in coefficient of thermal expansion was also observed. Simultaneous positive effects of CaSt(2) filler is attributed to its rich mesomorphic structure and its interaction with the iPP matrix. POLYM. ENG. SCI., 51:550-558, 2011. (C) 2010 Society of Plastics Engineers