Abstract: Steam flooding is a widely used technique to enhance oil recovery of heavy oil. Thermal viscosity reduction and distillation effect are considered as two main displacement mechanisms in steam flooding process. However, the molecular composition understanding and contribution for oil production are still unclear. In this study, the composition analysis of the heavy oil was investigated in the core scale steam flooding process with the temperature from 120 °C to 280 °C. The crude oil, produced oils and residual oils were characterized comprehensively by gas chromatography and high-resolution mass spectrometry. It is found that steam flooding preferentially extracts aromatics and remains more resins in the residual oil. Viscosity reduction is the dominant mechanism when steam is injected at a low temperature. Large molecular heteroatoms with high carbon number and high double bond equivalent (DBE) are eluted into the produced oil, while compounds with low carbon number and low DBE are remained in the residual oil. As the steam temperature rises, the increased distillation effect results in the extraction of light hydrocarbons from the residual oil to the produced oil. More small heteroatoms with low carbon number and low DBE enter into the produced oil, especially in the none water cut stage. The compositional difference of produced oils is characterized in DBE versus carbon number distribution of the N and O containing compound classes. This work uses a variety of composition analysis methods to clarify the steam flooding mechanism and provides a novel understanding of steam flooding mechanisms with various temperatures and production stages from the molecular perspective.