To understand annual variation of abundance and biomass of planktonic ciliates in the Bohai Bay, ciliates were sampled monthly at a fixed station from July 2019 to June 2020. A 1-L sample of surface water was collected with a 2.5 L Niskin water sampler and fixed in 1% acid Lugol's iodine solution. Water samples were pre-concentrated using the Utermöhl method and observed under an Olympus IX71 inverted microscope at 100×or 400×magnification. The dimensions of the ciliates were measured and the cell volume of each species was estimated using appropriate geometric shapes. The carbon:volume ratio used to calculate biomass was 0.19 pg C/μm³. Aloricate ciliates were divided into three classes according to the size of cell volume. The classification of tintinnids was based on taxonomic literature. The average abundance of aloricate ciliates was (1382±1929) ind/L, ranging from 137 ind/L to 6748 ind/L. The average biomass of aloricate ciliates was (2.29±3.05) μg C/L, ranging from 0.09 μg C/L to 10.83 μg C/L. The average tintinnid abundance was (946±2384) ind/L, ranging from 0 ind/L to 8440 ind/L. The average tintinnid biomass was (3.86±11.57) μg C/L, ranging from 0.00 μg C/L to 40.57 μg C/L. The average abundance of total ciliates was (2328±2735) ind/L, ranging from 143 ind/L to 8577 ind/L. The average biomass of total ciliates was (6.15±11.35) μg C/L, ranging from 0.09 μg C/L to 41.02 μg C/L. Both abundance and biomass of aloricate ciliates and tintinnids showed strong annual variations throughout the year. Two peaks of aloricate ciliates were observed in April and August, respectively. Tintinnid abundance and biomass peaked in July. Tintinnids occupied (28.6±32.6)% of total ciliate abundance, being higher (>50%) from May to July. Twenty one tintinnid species were identified, 13 of which were in genus Tintinnopsis. The dominant species were Leprotintinnus simplex, Leprotintinnus nordqvisti and Tintinnidium primitivum. Species number of tintinnids showed higher level from June to August. Species composition of tintinnids exhibited obvious annual variation, and temperature was the main environmental factor driving the annual variation of tintinnid community. The average value of the Shannon index and Pielou index of tintinnid community were 0.95±0.78 and 0.52±0.34, respectively. Both Shannon index and Pielou index of tintinnid community were higher from June to August. There were not significant relationships between aloricate ciliate abundance and environmental factors. Tintinnid abundance, total ciliate abundance, and the percentage of tintinnids to total ciliate abundance were significantly positively correlated with temperature and Chl a concentration, while negatively correlated with salinity.