The knowledge of solid-state second phase heterogeneous nucleation process is limited due to the experimental difficulty, such as tiny length scale, short time period, and high temperature condition. In recent years, some significant breakthroughs in nucleation studies have been achieved by aid of computational techniques. In this study, we apply molecular dynamics (MD) simulations to perform with heterogeneous nucleation occurring at grain boundaries (GB) during the austenite (FCC) phase to ferrite (BCC) phase transformation in a pure Fe polycrystalline system. A neighbor vector analysis (NVA) method has been introduced and it is shown how the NVA can be used to determine the misorientation of grain or interphase boundaries, which allow a further investigation of the boundary structure correlated to interfacial energy and mobility during the nucleation and early grain growth stage. Meanwhile, benefited from the MD technique, the bulk energy, grain boundary energy, and interfacial energy can be individually captured during the simulations, which allow a detail analyze of the shape, critical size and nucleation energy of specific nuclei, through the classical nucleation theory (CNT) and according to a faceted-spherical cap geometric model (FSC). In addition, we also compared the results from the classical approach with a new algorithm that combination of the multi-phase field model (MPFM) and the nudged elastic band (NEB) method to demonstrate the CNT in the solid-state conduction. Finally, we extend our simulation method to a more complex triple GB junction nucleation event, and investigate the non-classical barrier-free nucleation behaviors. The results support the critical informations to clarify the initial state of austenite to ferrite transition, and improve our knowledge of the heterogeneous nucleation process, which help to bridge the gap between the experimental measurements and the theoretical calculations. The simulation method also provided a new approach for studying the complicate heterogeneous nucleation phenomenon in solid-state for a wide variety of polycrystalline material systems. / Thesis / Doctor of Philosophy (PhD)