Using　contact　measurements　for　small　modulus　gears　is　difficult　owing　to　the　limited　tooth　spaces.　This　work　focuses　on　the　vision　measurement　of　small　modulus　gears　with　unknown　parameters.　Based　on　the　sub-pixel　technique　used　in　digital　image　processing,　the　gear　measurement　datum,　which　is　the　geometric　center　of　the　gear,　is　located.　Thereafter,　the　tooth　number,　modulus,　diameter　of　the　tooth　addendum　circle,　and　diameter　of　the　tooth　dedendum　circle　are　calculated.　According　to　the　definition　of　gear　accuracy　in　ISO　1328-1:　2013,　this　study　introduces　an　evaluation　method　and　a　measurement　process　for　gear　pitch　deviations　and　profile　deviations　based　on　visual　measurements,　and　an　evaluation　software　is　developed.　A　visual　measurement　test　on　the　pitch　and　profile　deviations　of　involute　cylindrical　spur　gears　with　a　modulus　of　0.5　mm　was　carried　out,　and　the　results　were　compared　with　those　from　a　gear　measurement　center.　The　comparison　indicated　that　the　absolute　error　in　the　left　and　right　tooth　surface　measurements　had　a　maximum　value　of　4　μm　and　a　minimum　value　of　1　μm;　the　gear　accuracy　for　both　cases　was　judged　to　be　grade　8.　This　vision　measurement　system　can　be　used　for　small　modulus　gears　within　a　certain　range.　©　2021,　Science　Press.　All　right　reserved.