DC-DC　buck　converters　with　high　conversion　efficiency　are　widely　used　in　ultralow　power　applications.　This　paper　proposes　a　new　method　of　power　metal-oxide-semiconductor　field-effect　transistors　(MOSFETs)　size　optimization　for　DC-DC　buck　converters　based　on　optimum　power　efficiency.　It　analyses　the　power　loss　of　DC-DC　buck　converters　and　presents　an　equation　describing　the　relationship　between　their　power　MOSFETs　size　and　their　power　efficiency.　In　addition,　a　method　is　proposed　for　optimizing　the　power　MOSFETs　size　of　DC-DC　buck　converters.　This　method　is　validated　by　simulation　result　from　MATLAB　and　implemented　in　a　0.35　μm　CMOS　process.　The　power　efficiency　of　the　DC-DC　buck　converter　was　scanned　for　different　power　MOSFETs　sizes　at　an　input　voltage　of　3.6　V　and　a　load　current　of　300　mA,　and　the　optimum　size　(based　on　optimum　power　efficiency)　is　obtained.　The　results　of　this　simulation　verify　the　accuracy　of　the　power　MOSFETs　size　optimization　methods.　©　2019　IEEE.