Hydrophobic and high-molecular-weight dissolved organic matter (DOM) can be efficiently removed via coagulation, but DOM with a low hydrophobicity and low molecular weight cannot. These can be respectively classified as high-coagulability organic matter (HCOM) and low-coagulability organic matter (LCOM). Most coagulation studies have focused on HCOM (such as humic acid) but not LCOM. Therefore, the coagulation behavior and mechanism of LCOM were revealed by comparing with HCOM in this study. The removal efficiency of HCOM via coagulation was high (>75%), while that of LCOM was very low (<15%). Charge neutralization (CN) and sweep flocculation (SF) were the primary mechanisms of DOM coagulation. Based on the results of fluorescence measurements, zeta potential, and floc growth processes, the CN mechanism was very efficient in HCOM coagulation, but it did not apply to LCOM coagulation. Since LCOM has a smaller molecular size, it bound with coagulants to form soluble complexes in contrast to the flocs formed during HCOM coagulation. Both HCOM and LCOM could be removed via the SF mechanism, but the interaction between LCOM and coagulant was far weaker. According to the analysis of floc properties, HCOM strongly bound with the coagulant to form flocs with a loose structure and uneven surface. The properties of LCOM flocs were very similar to those of hydroxide flocs formed without organics in terms of surface charges, fractal structure, recoverability, surface morphology, and contents of Al/O components of flocs. In conclusion, HCOM coagulation was controlled by the interaction between HCOM and coagulant, while LCOM coagulation was dominated by coagulant hydrolysis and polymerization. This study complements the existing understanding of LCOM coagulation and provides insights into the enhancement strategy.