Timely detection of herbicide resistance at an early stage of crop cultivation is essential to help farmers find alternative solutions to manage herbicide resistance in their fields. In this study, maximum quantum yield of PS II [F v /F m = (F m -F o )/F m ] was measured at the 4-5 leaf stage to discriminate between herbicideresistant and susceptible biotypes of Echinochloa species. The differences in F v /F m between herbicide-resistant and susceptible Echinochloa spp. were consistent with the whole-plant assay based on I 50 (herbicide doses causing a 50% inhibition of F v /F m ) and GR 50 (herbicide doses causing a 50% reduction in plant fresh weight) values and R/S ratios (herbicide resistance index), regardless of the mode of action of the tested herbicides. A PS II inhibitor caused the fastest inhibition of F v /F m , compared with ACCase and ALS inhibitors, after herbicide treatment. The required time for discrimination between herbicide-resistant and susceptible Echinochloa spp. was 64 h after PS II inhibitor treatment, much shorter than those of ACCase and ALS inhibitor-treated plants, which required 168 and 192 h respectively. The leaf chlorophyll fluorescence assay provided reliable diagnostics of herbicide resistance in Echinochloa spp. with significant time savings and convenient measurement in field conditions compared with the conventional whole-plant assay.