Intermittent Cycle Extended Aeration System (ICEAS) offers advantages for treating sewage; such as easy operation, process flexibility, and low capital cost. A laboratory-scale study was made with synthetic-domestic wastewater (COD=300 mg dm(-3); BOD=210 mg dm(-3)) to investigate appropriate conditions for reduced operating cost. The results from this study indicated that the maximum hydraulic loading and organic loading were 3.5m(3)m(-3)d(-1) and 0.735kg BOD m(-3)d(-1) respectively. The BOD and COD of effluent were 15.5 mg dm(-3) and 29.6 mgdm(-3) for the cycle time and aeration time of 3.4h and 2.65h. It was not necessary to supply external artificial substrates in the reactor to deal with low wastewater flow that caused the starvation of sludge. Specific oxygen uptake rate (SOUR) was used as the index of microbial activity. The study indicated that the microbial activity could be restored (SOUR=20.5mgg(-1) MLVSS h(-1)) after 5-6 days of cultivation when the sludge was deprived of substrate for 17 days.