A laboratory experiment was carried out to in-vestigate the sedimentation characteristics of Tondano lake clay and effect of gypsum addition, included the settling patterns as the previous experiments. Firstly, the floccu-lation and dispersion characteristics, which are evaluated by turbidity, are substantially influenced by pH and salin-ity. Then, it was found that the addition of 5 % gyp-sum to this clay influenced the settling pattern and set-tling/consolidation rate. In the region in which consolida-tion settling does not occur (above 3000 % water content), the effects of gypsum addition on the settling patterns are significant; whereas in cases that the consolidation settling occurs (under 2000 % water content), the effects of gyp-sum addition are not so much significant. The greater wa-ter content of the clay suspension could induce a higher degree of flocculation, thus explaining sedimentation char-acteristics induced by gypsum treatment.