Highly lattice mismatched InP/Si nanowire heterostructures were synthesized using metal organic chemical vapor deposition (MOCVD) process at 450 ºC. The InP nanowire diameter as high as 500 nm is much thicker than the critical diameter (~24 nm for InP/Si) predicted by a recent theoretical work on the coherent growth of nanowire heterostructures. We investigated possible factors that lead to the unusually large diameters in a highly lattice mismatched material system. Dislocations formed at the interfacial plane of the heterostructure due to high lattice mismatch were found to contribute to the growth of nanowires with very large diameters. An extra pair of dislocation lines at the interfacial plane was found to support an increase in nanowire diameter by ~12 nm.