The proper use of polymers as conditioners is a critical aspect of dewatering processes. In sludge conditioning investigations, one of the objectives is to determine the effect of different chemical dosages on dewaterability. Dewaterability of conditioned sludge is often characterized exclusively by capillary suction time (CST) and time to filter (TTF). In this study, we investigate to correlate specific resistence to filtration (SRF) with CST and TTF on alum sludge with cationic polymer (C-7650) conditioning. Correlation coefficient (0.9325) of TTF on SRF was more closely related than that (0.8588) of CST. At 1.49, 1.89 and 2.6% of sludge concentrations, SRF measurements were 8.93 × 10 exp (13), 16.8 × 10 exp (13) and 16.8 × 10 exp (13) m/㎏. As the cake solids concentration increased from 1.49 to 2.6%, the optimal dose of the polymer increased from 4 to 60 ㎎/l. SRF measurements at optimal dosages of C-7650 were 3.48 × 10 exp (13), 2.32 × 10 exp (13) and 1.94 × 10 exp (13) m/㎏ respectively. Increasing sludge concentrations, measurements of SRF were decreased and dewaterability was increased.