Gas phase pyrolysis of benzene was performed at 1,123, 1,173, and 1,223 K using a flow type reactor to examine the mechanism of the initial stage of coke (poly-aromatic hydrocarbons; PAHs) formation. The pyrolysis products including hydrogen, tar, and coke were quantified as the function of benzene residence time using gas chromatography, gas chromatography-mass spectrometry, LDI-TOFMS, and elemental analysis to establish the carbon balance within 95 to 105% for all of the experiments. Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) was applied to determine individual components in coke. Main tar components quantified were biphenyl (C12H10), terphenyl isomers (C18H14), quaterphenyl isomers (C24H18), phenanthrene (C14H10), and triphenylene (C18H12). Phenanthrene was found to be formed by the H-abstraction-C2H2-addition (HACA) mechanism and the other components were judged to be formed by the phenyl addition (PA) and the phenyl addition/cyclization (PAC) mechanisms. One of the smallest molecules identified within the coke was dibenzo[fg,op]naphthacene (C24H14) that is formed from triphenylene by the PAC mechanism. Larger molecules formed by the PAC mechanism and the HACA mechanism were also identified as coke components at the initial stage of pyrolysis. The phenyl addition to PAHs by the PAC mechanism and the HACA mechanism were judged to be main pathways to form larger PAHs. Main reaction mechanisms at the early stage of coke formation were suggested based on the detailed experimental data.