The molecular function and fate of mRNAs are controlled by RNA-binding proteins (RBPs) that assemble with the mRNAs to messenger ribonucleoprotein particles (mRNPs). However, the identification of all interacting proteins of a specific mRNA is still very challenging. Based on the widely-used RNA tagging with MS2 aptamers for RNA visualization, we developed a novel method called RNA proximity biotinylation (RNA-BioID). Here, a proximity labeling enzyme (a biotin ligase or an ascorbate peroxidase) is tethered to the 3’-UTR of endogenous MS2-tagged RNAs. RNA-associated proteins can then be biotinylated in vivo and subsequently isolated using the biotin label. We have demonstrated the feasibility of this approach by characterizing the dynamic interactome of the conserved β-actin mRNA in mouse embryonic fibroblasts. Currently, we apply this approach to various mRNAs and long non-coding RNAs in polarized cells, iPSCs, and neurons to compare and understand the composition of different RNP complexes.