Due　to　the　small　size　and　harsh　transmission　conditions　of　small-module　gears,　it　is　very　difficult　to　measure　gear　characteristics　with　a　modulus　smaller　than　1　mm.　We　proposed　an　optical　coherence　tomography　(OCT)　method　for　measuring　small-module　gears.　Testing　of　a　30-tooth　copper　gear　with　a　small　modulus　of　0.5　mm　was　carried　out　for　the　measurement　of　its　modulus,　tooth　parameter,　tooth　number,　pressure　angle,　modification　coefficient,　and　tooth　thickness　by　using　OCT.　In　addition,　the　influencing　factors　on　the　measurement　were　discussed.　The　whole　teeth　profile　of　a　0.2　mm　modulus　gear　was　imaged　by　processing　of　the　data　collected　from　different　clamping　angles.　Compared　with　the　visual　imaging　without　depth　information　and　small-scale　microscopic　imaging,　the　OCT　method　has　shown　its　superiority,　and　it　has　potential　in　the　application　of　the　measurement　of　micro　gears　with　a　small　modulus　of　less　than　0.2　mm.　©　2018　by　the　authors.