Six (6) hot springs strains and two (2) Synechococcus strains from Universitas Indonesia have been observed to determine the maximum growth temperature of those strains. The strains were as follows: HS-1 (CIS001), HS-7 (CIS007), HS-8 (RDB001), HS-9 (RDB002), HS-13 (RDB006), HS-18 (PAN005), UI-56 (6-Ag7air) and UI-57 (9-Ag9air) strains. The eight strains were isolated from three (3) hot springs in West Java (Ciseeng, Rawa Danau Banten, and Pancar Mountain) and one (1) small lake in Universitas Indonesia (Agathis). The water temperature of habitats were ranges at 36-43 °C (Ciseeng), 35-50 °C (Rawa Danau Banten), 46-69 °C (Pancar Mountain), and 27-29 °C (Agathis small lake). Incubation temperature were 23±1 °C (A), 30±1 °C (B), 35 °C (C), and 50 °C (D), while the observed parameters were cell density during growing and chlorophyll content. Observations were conducted over a period of 42 days. The results showed that all of Synechococcus strains experienced growth phases, i.e. the adaptation (lag) phase, exponential (log) phase, stationary phase, and death phase. The average of cell density of Synechococcus strains decreased at T0-T1 and started to increase at T2 and T3. The highest cell density on the growth curve was found at around 14 days (T14) days for the temperatures of 20 °C, 30 °C, and 35 °C, while it was found at around 35 days (T35) for the temperature of the 50 °C. The highest chlorophyll content acquired during studies was differed from each strain. Overall the highest chlorophyll content occurred at the treatment temperature of 20 °C, 30 °C and 35 °C, and the lowest occurred at the treatment temperature of 50 °C. However, exception was observed on HS-1 (CIS001) strain grown at a temperature of 50 °C which experienced the highest levels of chlorophyll content at T42. The maximum growth temperature of Synechococcus strains were observed at temperatures between 30 °C and 35 °C.