Dose-response analysis is usually employed in the weed science study. Log-logistic model has been adopted widely in herbicide dose-response analysis. This model has biologistically meaningful parameters and can be easily solved by some statistical packages. In this paper the log-logistic model was reviewed in theory and a real data set was used to explain the log-logistic model. Glyphosate was diluted and applied in corn seed and purple nutsedage tuber. The germination rate was investigated within three days. Both SAS and SigmaPlot packages were used in analysis. It was found that the herbicide glyphosate was acting at the same site of action in corn seed and purple nutsedge tuber and two statistical packages achieved a similar result.