The nucleation of W-rich Laves phase nanoparticles was studied during long-term aging at elevated temperature in tempered martensite ferritic steels (TMFS). The TMFS was fabricated by vacuum induction melting (VIM) process. The long-term aging tests are interrupted at various stages to simulate the different level of nucleation of intermetallic phases. In the present work, we employ scanning electron microscopy (SEM), Auger electron microscopy (AES), and transmission electron microscopy (TEM) to study the formation nucleation of Laves phase particles. We investigated the preference of Laves phase particles to nucleate next to M23C6 micrograin boundary carbides due to the segregation of W from the matrix to the micrograin boundaries.