This study aimed to analyse the effect of toxic heavy metal on microalgae growth. Several microalgae i.e cyanophyceae (Spirulina maxima), eustigmatophyceae (Nannochloropsis oculata), chlorophyceae (Chlorella vulgaris) and porphyridiophyceae (Porphyridium cruentum) were exposed to mercury with various concentrations
(1, 3 and 5 mg. L-1). An experimental method was carried out in the laboratory scale with one control of microalgae culture without mercury exposure. The microalgae cultivated by using Walne medium with the initial cells were 10,000 cells.mL-1 for S. maxima and N. oculata respectively and 100,000 cells.mL-1 for C. vulgaris and P. cruentum respectively. The microalgae density was counted every day for 8 days. Water quality parameters were temperature, salinity, light intensity and pH. All treatments were set and calculated by using completely randomized design Anova and SPSS.16, respectively. The result depicts that the cell density of microalgae decreased during the culture except in control. A significant decay was shown by microalgae on exposure to 3 mg.L of mercury, where final density of microalgae on the 8th day was 2,333 cells.mL-1 (S. maxima), 13,000 cells.mL (N. oculata), 6,000 cells.mL-1 (C. vulgaris) and 2,000 cells.mL-1 (P. cruentum). However, all the quality indices are belonging to optimum condition for algae growth, such as temperature was 25-28°C, salinity was 34-38 ppt, light intensity was approximately 3,600 lux, and pH was 7-8. This research recommends to investigate the effect of heavy metals on ultrastructure of microalgae by using transmission electron microscope (TEM) and the chlorophyll content of microalgae contaminated.