To clarify the effect of added sludge char on the bio-drying rate and bio-drying behaviour of sludge, a composting experiment with a small composting unit and a drying experiment with an air-blow drier were carried out. In the composting experiment, sludge char manufactured from a compost of sludge was mixed with dewatering sludge, and the moisture content, volatile matter of the compost and CO2 production during the fermentation was measured. In the air-blow drying experiment, addition of char to sludge was found to increase the dewatering rate during the fermentation and to promot the decomposition of organic matter in the sludge. When the air-blow drying was applied during the fermentation, dewatering performance improved by adding the char, but, the decomposition of organic matter was inhibited. Char addition also improved the drying rate of sludge, which increased with increase in the amount of char added.