The abundance and species composition of ciliates were analyzed in the Bukan Dam Reservoir (West Azerbaijan, Iran) from January to December 2008. Surface water samples were collected at monthly intervals from the middle (Stations 1 and 3) and shallow (Stations 2 and 4) areas of the lake. Three samples were taken in the water column using a 5-litre sampler. Totally 30 liters water was collected, 200 ml of which was fixed immediately with 8.6 ml of a saturated HgCl2 solution and stained with 0.04% bromophenol blue. Three subsamples were counted in a 5-ml chamber and examined with a microscope (100 x) as well as Chlorophyll a and a few ecological parameters such as dissolved oxygen, pH, transperancy and water temperature were measured. Totally 50 ciliates species were found in Bukan Dam Reservoir. Regarding the density, a high-density period was detected from January to April with a mean density of 2.86 x 103 cells.l-1 Ciliata at stations 1 and 3 and 4.16 x 103 cells.l-1 at stations 2 and 4. The high density of ciliate in summer may be due to the increase in the density of bacteria and moderate metazooplankton as a result of the appearance of non-edible algae . The ciliata occurring at the highest densities were Coleps tessellates, Didinium nasutum, Paradileptus elephantinus, Stentor polymorphus, Zosterograptus labiatus, Paramecium bursaria, Cyclidium citrullus, Vorticella campanula, Halteria grandinella and Aspidisca costata. The maximum and minimum chlorophyll a values observed at the reservoir water were 10.39 µg.l-l and 1.0 µg.l-l being obtained in May and Octobor respectively.