Avermectin and milbemycin are important 16-membered macrolides that have been widely used as pesticides in agriculture. However， the wide use of these pesticides inevitably causes serious drug resistance， it is therefore imperative to develop new avermectin and milbemycin analogs. The biosynthetic gene clusters of avermectin and milbemycin have been identified and the biosynthetic pathways have been elucidated. Combinatorial biosynthesis by domain swap provides an efficient strategy to generate chemical diversity according to the module polyketide synthase (PKS) assembly line.