The objective of this study is to summarize the biomass conversion pathway through analyzing the changing trend of oil species and revealing the relationship between the evolution of oil species and the evolved gas composition at varied temperatures. For this purpose, the pyrolysis of palm oil wastes was conducted in a countercurrent fixed bed reactor at different final temperatures 500 to 900 degrees C, and the pyrolysis oil was thoroughly identified using various approaches including CNHS/O elemental analysis, Fourier transform infrared spectroscopy, column chromatography, and gas chromatography/mass spectrometry analyses. With the temperature increasing from 500 to 900 degrees C, the yield of oil was decreased while that of the total gas was enhanced greatly. Meanwhile, the variety and proportion of oxygenated compounds inside the oil gradually declined with an increase in the temperature, whereas those of the polycyclic aromatic hydrocarbons (PAHs) showed a marked increase, with a peak observed at 800 degrees C. The changing trend of the oil main components versus the temperature could be concluded as follows: with increasing temperature, the primary oil, containing mainly oxygenated compounds, was transformed into a secondary oil consisting of dominant phenolics; then, the oil became more aromatic with rising temperature and finally led to the tertiary oil that was dominated by PAHs. Meanwhile, the increasing temperature promoted secondary reactions of oil to result in the evolution of gases such as CO, CO2, H-2, CH4, and so forth. In particular, the yield of H-2 and CO could be regarded as an indicator for secondary reactions of oil.