A simulation test was conducted in aquaria to study the responses of algal communities to different N/P ratios in urban water body. The water sample was taken from a small artificial lake in Tianjin, and its initial N/P ratio was adjusted to 0. 5:1,7.2:1, 25:1 and 50:1, respectively. The results showed that in high N/P ratio groups, the numbers of Chlorophyta species decreased, while those of Cyanophyta species didn' t change very much. The numbers of these two species were both decreased in low N/P ratio group. Algal biomass, cell density and chlorophyll a content in medium and high N groups were higher than those in control and high P groups. The mean value of chlorophyll a reached the highest (69.7 microg x L(-1)) in high N group, and was 54.3, 30.3 and 29.7 microg x L(-1) in medium N, control, and high P groups, respectively. At the mid-late stages of culture, green algae Dictyosphaerium pulchellum was dominant in high P group, while blue algae Phormidium tenue, P. corium, Lyngbya limnetica and Microcystis aeruginosa were dominated in high N/P ratio groups. Control group had the highest species richness, while medium and high N groups had the highest and lowest ecological species dominance, respectively.