Additive　manufacturing　(AM),　also　known　as　3D　printing　technology,　is　applied　to　fabricate　complex　fin　structures　for　heat　transfer　enhancement　at　inner　surface　of　tubes,　which　conventional　manufacturing　technology　cannot　make.　This　work　considered　rectangular　fins,　scale　fins,　and　delta　fins　with　staggered　alignment　at　the　inner　wall　of　heat　transfer　tubes　for　heat　transfer　enhancement　of　internal　flows.　Designed　fin　structures　are　trial-printed　using　plastic　material　to　exam　the　printability.　Laminar　flow　convective　heat　transfer　has　been　numerically　studied,　and　heat　transfer　performance　of　the　tubes　with　3D-printed　interrupted　fins　has　been　compared　to　that　with　conventional　straight　continued　fins.　The　benefit　from　heat　transfer　enhancement　and　the　loss　due　to　increased　pumping　pressure　is　evaluated　using　the　total　entropy　generation　rate　in　the　control　volume　of　heat　transfer　tube.　As　the　conclusion　of　the　study,　better　heat　transfer　tubes　with　3D-printed　internal　fins　are　recommended.