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In order to evaluate the effects of bentazon and nicosulfuron on the photosynthetic apparatus of X. strumarium in greenhouse condition, chlorophyll a fluorescence (ChlF) method was studied at 1, 2, 3, and 4 days after herbicide treatment (DAHT). Bentazon in comparison to nicosulfuron, dramatically affect ChlF parameters and the shape of polyphasic OJIP curves at 1 DAHT. Bentazon significantly decreased photosystem II activity via increasing minimum fluorescence (Fo) and maximum quantum yield of basal non-photochemical energy losses (Fo/FM) and decreasing maximum fluorescence (Fm), variable fluorescence (Fv), time to reach maximum fluorescence (Tfm), size of the plastoquinone pool on the reducing side of photosystem II (Area), maximum efficiency of water-splitting complex on the donor side of the photosystem II (Fv/Fo), maximum quantum efficiency of photosystem II (Fv/Fm), trapping flux leading to QA reduction per RC (TRo/RC), electron transport flux further than QA– per RC (ETo/RC), and electron flux reducing end electron acceptors at the photosystem I acceptor side per RC (REo/RC). In most cases, the inhibitory effects of bentazon on photosystem II activity were observed at 1 DAHT but, those inhibitory effects were delayed by nicosulfuron application. It was concluded that ChlF is a rapid method for studding herbicide mode of action as photosystem II activity considered primary sites of action in bentazon and secondary sites of action in nicosulfuron.