In order to study the sludge microstructure and dewatering performance, the sludge was treated using different conditioners, the same conditioner in a different dosage, the same formula in a different proportion and different order of adding conditioners. The microstructure of dehydrated sludge was observed by scanning electron microscopy. The relationship between the sludge dewatering properties and microstructure characteristics was discussed. It was proven that the different dehydration performance correspond to the different morphology. The better dewatering performance shows more pore at the surface of sludge cake.Contractly, the sludge cake with poor dewatering perform a dense surface, even membranous. Therefore, there is a certain of relationship between the microstructure of sludge cake and its dehydration performance.