Plant oils are excellent renewable raw materials for thermosetting polymers, especially epoxy resins. The presence of double bonds, hydroxyl groups, and carboxyl groups in triglycerides allows for the attachment of some functional groups through chemical modification. Here we describe various chemical pathways for functionalizing triglycerides and fatty acids. Plant-oil-based epoxy resins exhibit the same excellent performance as petroleum-based epoxy resins, but burn easily, which limits their application in high-speed rail, aircraft, construction, and other fields. Therefore, imparting flame retardancy to vegetable-oil-based epoxy resins is very important. This chapter aims to summarize the most relevant and recent advances in intrinsically flame-retardant plant-oil-based epoxy resins. First, the synthesis method of a plant-oil-based flame retardant epoxy monomer is briefly introduced. Then special attention is paid to the flame retardancy, mechanical properties, and thermal stability of some plant-oil-based flame retardant epoxy resin thermosets of plant-oil-based epoxy monomers or curing agents. Finally, we briefly comment on the opportunities and challenges for the future development of plant-oil-based flame retardant epoxy thermosets.