The aim of the study was to determine the diversity and the distribution of the phytoplankton of Mefou Lake of Yaounde. The study was conducted from May 2019 to May 2020. The pelagic area was sampled in seven stations. A total of 79 species were identified, including approximately 29.87% of Cyanophyceae, 18.18% of Bacillariophyceae, 14.28% of Chlorophyceae and Euglenophyceae respectively, 06.49% Mediophyceae, 03.89% of Dinophyceae, 02.59% of Conjugatophyceae, Coscinodiscophyceae, Cryptophyceae, and Trebouxiophyceae respectively, 01.29% of Pyramimonadophyceae and Ulvophyceae respectively. Cyanophyceae are the most common and abundant class in samples taken from lake stations. The genus Eudorina is common to all stations. The transparency of the water varies between 0.35 m and 0.40 m in the stations. Mefou Lake has a uniform eutrophic state and poor ecological status.