We performed dilution experiments together with copepod added incubations to examine the influence of mesozooplankton on the grazing pressure of planktonic ciliates in Sihwa lake during summer when the abundances of phytoplankton and mesozooplankton increased considerably. Planktonic ciliates consumed 104% of primary production in a day on dilution experiments. However, the ciliates consumption on phytoplankton was reduced to 19% in copepod incubations with Acartia sinjiensis added. This was due to selective predation of A. sinjiensis on oligotrich ciliates ($>20{mu}m$) which were major grazers on nano-phytoplankton. Our experiments show that grazing pressure of planktonic ciliates based on dilution experiments may be overestimated when the abundance of planktonic ciliates is strongly controlled by copepods. We postulate that the role of planktonic ciliates as grazers could diminish in Sihwa lake in spring and summer when abundance of copepods increase considerably. We suggest that the predation of mesozooplankton should be considered to better appraise the role of planktonic ciliates as grazers.