In this study, the individual (alkaline: 1 ~ 10 g KOH/L sludge, ultrasonic: 500 ~ 50,000 kJ/kg TS, thermal: 80 ~ 160℃, and microwave: 100 ~ 180℃) and combined (alkaline + ultrasonic; alkaline + thermal; and alkaline + microwave) pretreatment effect on sewage sludge solubilization was investigated. While the solubilization (SCOD/TCOD) increase was limited to 50% in individual pretreatment, it was enhanced over 70% in combined pretreatment. In (alkaline + ultrasonic) pretreatment, 70% of solubilization was achieved in most of experimental conditions. The increase of alkaline pretreatment intensity up to 4 g KOH/L sludge was found to be efficient to lower the energy requirement in the following ultrasonic pretreatment. Also, the synergistic solubilization effect was found at < 4 g KOH/L sludge condition. However, further solubilization increase was negligible at over (4 g KOH/L sludge + 12,000 kJ/kg TS) of pretreatment condition. In (alkaline + thermal) and (alkaline + microwave) pretreatment, solubilization increased as the combined intensity increased. The synergistic solubilization effect was observed at (6 g KOH/L sludge + 90 ~ 130℃) pretreatment condition, and it kept increased with the increase of alkaline dosage. However, in (alkaline + microwave) pretreatment, the lowest solubilization and no synergistic solubilization effect was found. These results can be efficiently used for the economical application of sludge pretreatment.