Although sludge conditioning using chemicals has been employed widely to improve the thickening and dewatering processes and considered as is a good alternative for process improvement, however, its cost is high. Therefore, it is important to find effective and cheap conditioners. Large quantities of cement kiln dust (CKD) – from cement factories, and fly ash (FA) – from power plants, are promising conditioning agents. Accordingly, the experimental work presented in this study was designed to investigate the effect of using these by-products and their mixture on the thickening and dewatering performance of different types of sewage and drinking water sludge. The results show that the mixture of the CKD and FA had the higher effect and it increased the settling velocity, hydraulic loading rate and solid loading rate of all tested sludge types. The percent increase ranged between 67% and 225%, the dewaterability of the treated and untreated sludge was investigated by gravitational settling test, specific resistance to filtration with indicators for drying beds – solid loading rate and dewatering time and indicators of mechanical dewatering yield. Dewaterability tests show that using the CKD is more effective than the FA or the mixture for dewatering and it could decrease the drying time by values ranged from 37% to 87%.