The aim is to treat oily sludge from floatation process of oil produced water by deflocculation. Based upon analyzing the granularity distribution and dewater efficiency of the flocs, floc breakage and dewater of oily sludge was carried out. The results indicated that the coagulant skeleton builder in sludge could be broken by changing the coalescent state, and deflocculation could disrupt skeleton of the floc and release interstitial water in sludge. The highest dewatering efficiency is about 81.2% at the pH of 4.5 by gravity sedimentation. The sludge had a broader range of granularity distribution and a smaller volume-average diameter. Moreover, the median diameter d (0.5) of the treated sludge was 36.0 μm, which was smaller than 73.1 μm of raw sample. Meanwhile, capillary suction time (CST) significantly decreased with pH adjustment. A wastewater removal efficiency of 90.99% for the treated sludge by pressure filtration could be achieved. The method would be a potential technique to be applied to other kinds of floatation sludge.