To identify the seasonal variability of body-size spectrum for monitoring surveys based on periphytic protozoa, a one-year baseline survey was carried out in a coastal region of Yellow Sea, northern China. A total of 240 glass slides were collected after immersion times of 3, 7, 10, 14, 21 and 28 days in a four season cycle, i.e., winter, spring, summer, and autumn. Body-size ranks S2 and S5 dominated the periphytic protozoan communities from the initial stage (from day 3) to the next periods in spring and autumn, while body-size ranks S7, S8 and S4 showed high variety at the equilibrium stages (from day 10) in summer and winter. The expectation analysis revealed that the samples had different patterns of departure from the anticipated body-size spectrum in each season. This study shows that an ideal sampling approach needs to be established when protozoa is used as bioindicators of marine water quality.