Abstract: Background and Objective: Mercury is the most toxic heavy metal found in the environment. All aquatic organism has the potential to accumulate heavy metal on their body including Tubifex sp. Metallothionein has an important role in heavy metal detoxification mechanism which synthesized in each organism body. The aim of this study was to observe the mortality percentage, mercury (HgCl2) levels and mercury metallothionein (Hg-MT) levels in Tubifex sp. after 7 days of exposure at sub-chronic doses. Materials and Methods: The experimental method was used in this study. The mercury different sub-chronic doses (0, 12.5, 25, 37.5, 50 and 100%) were treated to Tubifex sp. triplicate for accumulative 7 consecutive days. Results: The result showed that the highest mortality (%) of Tubifex sp. was 100% sub-chronic dosage of mercury exposure with value 98.7±1.2% and the lowest mortality (%) was 0% sub-chronic dosage of mercury exposure with value 16.7±1.5%. The highest level of mercury and Hg-MT were in the 100% sub-chronic dosage of mercury exposure with value 4930±88.9 and 0.117±0.011 μg L–1, respectively and the lowest level were 0% sub-chronic dosage of mercury exposure with value 13.3±5.8 and 0.019±0.009 μg L–1, respectively. The Hg-MT level was not significantly different between 100 and 50% of sub-chronic doses, then the mortality (%) and HgCl2 level in Tubifex sp. body were significantly different in 100% compared to all treatment. Conclusion: So, it was concluded that the maximum capability of Tubifex sp. in synthesizing Hg-MT was 50% sub-chronic dosage of mercury exposure statistically with the highest mortality (%) and mercury level in Tubifex sp. was 100% sub-chronic dosage of mercury exposure.