The aim of this study is to investigate the variation of thermal conversion of agricultural wastes in a fixed bed reactor with temperature and elucidating the chemical structure of oil products formed during the process. Pyrolysis of two similar biomass samples, namely wheat straw (WS) and oat straw (OS), was conducted in a fixed-bed reactor with a heating rate of 7 degrees C/min, mean particle size, and pyrolysis temperature ranging from 300 to 800 degrees C. Influence of pyrolysis temperature on product yields and compositions was investigated. The results indicated that the maximum oil yields were obtained at moderate temperatures (500-600 degrees C) for WS and OS as 19.1% and 20.3%, respectively. According to the column chromatography results, polar, aliphatic, and aromatic fractions were maximized in lower, moderate, and higher temperatures, respectively. The structure of oil was then explained in detail by analyzing with H-1 nuclear magnetic resonance (NMR) and gas chromatography/mass spectrometry (GC/MS) methods. The pyrolysis oils were found to be chemically highly heterogeneous and contained phenols, carbonyl groups, carboxylic acids, aromatic compounds, monoaromatic compounds, PAH's, esters, n-alkanes and alkenes, and nitrogenous compounds. According to these results, oils can be used as fuel and are a valuable source of chemical raw materails.