The thecate dinoflagellate bloom microalga, Scrippsiella trochoidea, was co-cultured with different grams of fresh macroalga Gracilaria lemaneiformis under laboratory conditions, to characterize the allelopathic inhibition effect of the seaweed on photophysiological activities of the microalga. Cell numbers was measured, and Chlorophyll a (Chl a) fluorescence transient O-J-I-P curves associated with its specific parameters were determined. Both concentration and time-dependent inhibition of S. trochoidea were observed when the fresh seaweed was added. The cell numbers and pigments contents were markedly decreased; the O-J-I-P curve coupled with its specific parameters was reduced. The inhibitory effects of the macroalga on the microalga, according to the JIP-test and pigments contents, include a decrease in the number of active reaction centers, the blocking-up of the electron transport chain. This study suggests that fresh G. lemaneiformis is effective in inhibiting photophysiological activity of S. trochoidea, and thus be a potential 'tool alga' for controlling S. trochoidea blooms.