In order to examine the optimal design and operating parameters in composting of waste activated sludge, a series of experiments were conducted to discuss the optimal operating parameters for aerobic composting of waste activated sludge obtained from Vinh Loc Industrial Park that is located in Ho Chi Minh City. The study was done by optimizing the ratios of sludge/inactive fillers and sludge/microbial products to improve the composting efficiency. After 50 days of composting, the composts of all experiment batches met the microorganism-organic fertilizer standards of 10 TCN 526:2002. The study indicated that the highest efficiency decomposition was obtained at the ratio of a sludge/inactive fillers of 1/0.1 and sludge/microbial products of 1/0.05: TOC (Total Organic Carbon) reduced by 20.2%, loss of Nitrogen was trivial (0.53%), the highest temperature reached at 55.5 o C and lasting for 5 days which ensures elimination of pathogens. However, the addition of nitrogen compounds during the composting is necessary to ensure the compost product can meet the nitrogen standard of microorganism-organic fertilizer.