Block copolymers (BCPs) can self-assemble at the interface between two immiscible phases, with each block segregating into its compatible phase. The interfacial assemblies of BCP can effectively screen the unfavorable interaction between the phases, significantly reducing interfacial tension. The phase can vary from small molecule (e.g., water and oil) to macromolecules (e.g., homopolymer). In Chapter 1 and Appendix A, we first presented the in-situ conversion strategy, where the hydrophobic-hydrophobic BCPs are converted to hydrophobic-hydrophilic BCP directly at the water-oil interface, leading to significant reduction in interfacial tension. In Chapter 2 and Appendix B, by employing in situ conversion with star BCP architecture, we achieved self-wrinkles at fluids interface. The wrinkle patterns were programmed by controlling the interfacial assembly of star BCP. In Chapter 3 and Appendix C, the phase behaviors of star BCP at fluids interface was studied by transferring the interfacial assemblies with silicon wafers. The morphology of transferred film was found to be highly dependent on the in situ conversion degree, molecular weight, volume fraction of each block and the BCP architecture. At homopolymer-homopolymer interface, BCP act as efficient compatibilizers. In Chapter 4 and Appendix D, we systemically investigated the interfacial conformation and compatibilization of star BCP at homopolymer interface. We revealed that increasing the arm numbers can enhance the binding energy per molecule, leading to higher compatibilization efficiency. In Chapter 5 and Appendix E, we developed Janus bottlebrush compatibilizers, consisting of two homopolymer chains randomly distributed along the linear backbone. At equilibrium, these side chains orient in opposite directions, maximizing binding energy while minimizing steric hindrance. In Chapter 6 and Appendix F, we also investigated the self-assemblies of bottlebrush BCP in thin films, showing the thin film morphology was highly dependent on the architecture (including degree of polymerization of backbone and grafting density) and the depth of the thin film.