The object of this study is to investigate the sintering mechanism of lightweight ceramic by different sintering temperature. The raw materials were sewage sludge and fly-ash, and clay was added as cementing agents (by ratio of 50wt% in each sample). The raw materials were mixed and pressed into raw pellets. After drying and preheating treatment, the raw pellets were sintered at temperatures ranging from 1050 to 1150 , in 25 increments for 10min. The physical properties (bulk density, grain density, water absorption and rate of expansion) were the indexes used to determine the technical parameters for the preparation of lightweight ceramic pellets and investigate the sintering mechanism of bloating. The results suggested that temperature was the key control factor of sintering process, and 1150 was the starting point of bloating reaction. Observation of the microstructure by SEM indicated that a higher flux content in the raw materials could lower the melt point during sintering process.