Abstract: A study on species diversity of phytoplankton and its relationship to the lake water quality was conducted at Lake Chini, Pahang. Sampling was conducted twice i.e. 10th. July 1999 which is commonly dry period for tropical and 11th. December 1999, which is wet season. Quantitative and qualitative sampling was done using the plankton net at the nine selected sampling stations. A total of 81 genus which is consists of 135 species of Bacillariophyta, Cholorphyta, Cyanophyta, Chrysophyta, Euglenophyta and Pyrrophyta division had been identified. Chlorophyta was quantitatively and qualitatively the most dominants division, which was dominated by genus Staurastrum spp, Cosmarium spp and Ankistrodesmus falcatus (Corda) Ralfs. From the quantitative sampling, the density of phytoplankton had an average of 2129 ind./ml, and one way ANOVA analysis ( = 0.05) showed that there is a significant differences between sampling stations, but not significantly different between two season. The Shannon diversity index of phytoplankton was detected ranging between 2.050 to 2.905 and one way ANOVA (( = 0.05) analysis showed no significant different of this index between season. The water quality of the Lake Chini was within the natural concentration. Only nitrate was detected slightly higher than the natural range ie ranging between 0.9 mg/l to 1.4 mg/l. The correlation test (( = 0.05) showed that the organisms density and diversity index did not have a significant linear correlation with the physical and chemical water quality. Ankistrodesmus falcatus (Corda) Ralfs. found highly abundance in both sampling and has a potential to be used as a biological indicator for the high level of nitrate-nitrogen, however, further study need to be conducted for confirmation.