Current　studies　indicate　that　the　axial　compressive　strength　of　circular　concrete-filled　steel　tube　columns　can　be　enhanced　by　embedding　circular　FRP　tubes.　However,　this　strength　enhancement　would　diminish　when　using　FRP　tubes　with　smaller　thickness　or　diameter.　Because　of　a　lack　of　adequate　research,　the　threshold　at　which　the　strength　enhancement　effect　starts　to　dominate　remains　unclear.　This　threshold　is　important　for　engineers　because　it　can　be　established　as　the　minimum　strength　enhancement　limit　in　designing　circular　concrete-filled　steel　tube　columns　with　embedded　circular　FRP　tubes.　To　address　this　knowledge　gap,　this　paper　conducted　comprehensive　parametric　analyses　by　using　the　benchmarked　finite　element　models.　The　analytical　parametric　results　were　obtained　and　used　to　quantify　the　relationship　between　the　strength　enhancement　ratio　and　the　relative　strength　factor　(i.e.,　Eq.　8　in　this　paper).　Based　on　this　relationship,　a　unified　design　equation　was　proposed　to　estimate　the　axial　compressive　strength　of　circular　concrete-filled　steel　tube　columns　with　embedded　circular　FRP　tubes.　It　is　calculated　as　the　axial　compressive　strength　of　circular　CFT　columns　using　the　AIJ　design　equation　multiplied　by　the　strength　enhancement　factor.　The　proposed　design　equation　offers　more　accurate　strength　predictions　than　the　current　design　equation　and　covers　a　wider　range　of　material　strength　and　geometric　parameters　(e.g.,　steel　yield　stress　and　concrete　compressive　strength　up　to　960　MPa　and　120　MPa,　respectively).