An　automatic　counting　method　for　microalgal　cells　were　proposed　using　digital　image　processing　with　characteristics　of　microalgae　microscopic　image　considered.　Firstly,　the　microalgae　image　was　pretreated　with　graying,　and　was　then　applied　the　median　filtering　to　remove　the　noise.　Secondly,　the　image　was　applied　Bot-hat　conversion　to　enhance　the　contrast.　Thirdly,　the　image　was　segmented　with　Otsu　algorithm,　and　was　then　applied　morphological　operations.　Finally,　the　binary　image　segments　representing　microalgal　cells　were　labeled　and　counted.　The　results　of　experiments　showed　that　this　method　was　simple,　efficient　and　accurate　in　counting　microalgal　cells　in　the　microscopic　image.　©　(2014)　Trans　Tech　Publications,　Switzerland.