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Chlorophyll fluorescence is a type of radiation emitted by plants as unused part of energy absorbed from sunlight. Its intensity depends on many stress factors, nutrition, weather conditions etc. Methods based on chlorophyll fluorescence measurement have potential to be perspective for the assessment of herbicide efficacy, phytotoxicity or stress influence. Two fluorometers (FluorCam and handy FluorPen) were used for measurements in laboratory experiments in 2007–2009. Herbicide Basagran super with active ingredient bentazone (480 g.l−1) was tested using a parameter called ‚Quantum yield of fluorescence‘ (QY) in three different doses: the registered dose 2.0 l.ha−1 (100%) and lower doses 1.5 l.ha−1 (75%) and 1.0 l.ha−1 (50%). Treated experimental plants of redroot pigweed (Amaranthus retroflexus L.) were measured in 0, 1, 2, 3, 6, 9 and 15 days after treatment and the results were compared with subjective assessment (estimation) and growth parameters. The obtained results showed that the effect of bentazone measured by both devices was statistically significantly different from untreated control. We can also detect herbicide effect earlier by QY measurement than by other methods. Different doses showed significant differences 15 days after treatment for dose 2.0 l.ha−1 in comparison with both reduced doses. The subjective assessment showed significant differences in all terms of measurement (1–15 days after application) except of the day of application. Regeneration of plants for which the youngest leaves were not treated by herbicide was observed for all doses. Difference of QY values between used devices was on average 0.119 (higher for FluorCam) but statistically insignificant.