Abstract: The experiment was conducted in the green house of Wageningen in the Netherlands in order to determine the minimum lethal herbicide dosage (MLHD) of photosynthesis-inhibiting herbicide,bentazone and terbuthylazine,on six weed species with plant photosynthetic meter (PPM). The results showed that the diagnostics of weed caused by bentazone could be observed 2 days after treatment and diagnostics of weed caused by terbuthylazine could be observed 8 days after treatment, and weed growth was inhibited later on. Both PPM values of weed leaves and weed biomass (shoot fresh weight) decreased as the herbicide dosage increased. However,the susceptibility of weed species to the tank mixture of bentazone and terbuthylazine was different among weed species.Chenopodium album,Solanum nigrum,and Amaranthus retroflexus were very susceptible to the herbicides.Polygonum persicaria and Abutilon theophrasti were relatively less susceptible to the herbicides,and Lycopersicon esculentum was not susceptible to it.The relationship between reduction rates of weed biomass and PPM values of weed leaves 2,4,and 6 days after treatment was established.The relationship between reduction rates of weed biomass and PPM values 4 days after treatment was the best with the coefficient of determination of 0.924 7.It was concluded that PPM values of weed leaves after treatment could be utilized to predict weed control efficacy and could help to calculate MLHD. PPM value 4 days after treatment could give the best prediction of MLHD of the mixture of bentazone and terbuthylazine.