Hydrothermal liquefaction (HTL) is an environmentally friendly treatment of sewage sludge (SS) that has attracted a lot of attention. This study investigated the effect of microwave pretreatment power (180-900 W) on the yield of the products and compositions of bio-oil from HTL of SS. Microwave power had a noticeable effect on organic matter in SS, because increasing microwave power can improve the yield (increment ranges between 2.2 and 10.1 wt %) of bio-oil. It was observed that the highest bio-oil yield (35.4 wt %) was at 360 W and the calorific value of bio-oil was also increased to 26.29 MJ/kg. The gas chromatography mass spectrometry (GC-MS) analysis of bio-oil further indicated an increment in the yields of flammable substances (e.g., aliphatic hydrocarbons, alcohols, etc.) and the decrement in the nitrogen-containing heterocyclic compounds (e.g., pyridines, amides, etc.) and organic acid. Additionally, the carbon conversion rate and energy input and output of SS during HTL were studied. It was not economical for energy production with the microwave pretreatment. However, this research provides a novel strategy to achieve environmentally friendly SS treatment.