This study focused on the bloom-developing process of the giant jellyfish, Nemopilema nomurai, on phytoplankton and microzooplankton communities. Two repeated field observations on the jellyfish bloom were conducted in June 2012 and 2014 in the southern Yellow Sea where blooms of N. nomurai were frequently observed. We demonstrated that the bloom was made up of two stages, namely the developing stage and the mature stage. Total chlorophyll a increased and the concentrations of inorganic nutrients decreased during the developing stage, while both concentrations maintained stable and at low levels during the mature stage. Our analysis revealed that phosphate excreted by growing N. nomurai promoted the growth of phytoplankton at the developing stage. At the mature stage, size compositions of microzooplankton were altered and tended to be smaller via a top-down process, while phytoplankton compositions, affected mainly through a bottom-up process, shifted to be less diatoms and cryptophytes but more dinoflagellates. Display omitted •Elucidation on effects of jellyfish blooming process on plankton communities.•Jellyfish blooms consist of two stages, the developing stage and the mature stage.•Growing jellyfish compensated for P deficiency and promoted phytoplankton growth.•Jellyfish blooms result in smaller sizes of microzooplankton via top-down control.•Jellyfish blooms lead to less diatoms and cryptophytes but more dinoflagellates.