The purpose of the study is to examine the ability of pattern classification technology for identifying the quantity of duckweed. The study chose the duckweed from Xin-Hai-Qiao constructed wetlands in Taipei County as the study sample, and it applied the pattern classification technology to calculate the quantity of duckweed on top of water surface in a limited area. In this experiment, five covering densities were built to compare the duckweed-counting accuracies of pattern classification technology. The assumption is that duckweed leaves are mainly light green and dark green. Then light pixels and dark pixels were adopted to build two groups of Mixture Gaussian Models for duckweed identification. While using simplified Mixture Gaussian Model, without overlapping of duckweed leaves, the counting accuracies under five covering conditions were 95.7%, 76.8%, 68.2%, 82.1%, and 90%. On the bases of two Gaussians and 90% acceptance region, the duckweed counting accuracy will reach 80%.