In this paper an advanced fluid inclusion analysis technique was used to study the geochemical characteris-tics of hydrocarbon inclusions in the Yancheng depression. The results indicate that the source inputs come from lower aquatic algae and higher plants. The sedimentary environment was reducing and the salinity of the water medium was high. The hydrocarbons had been mature before they entered into the reservoirs. The petroleum filling history shows that at least two filling processes have occurred and the latter one was most significant. The filling periods were 55-12 Ma and 32-34 Ma, respectively. The hydrocarbons of the Yancheng depres-sion mainly came from Palaeozoic marine sapropelic materials, partly from the Taizhou or Pukou formation. Ex-tensional faults are the effective migration paths.