The polyacrylonitrile carbonization tar is discussed in detail by using gas chromatographic-mass spectrometer and electron paramagnetic resonance. It is a viscous liquid that constitutes complicated condensed components resulting from polymer decomposition. It was demonstrated that the predominant products were acrylonitrile dimer and trimer species with trifle cyclohexane and cyclopentane. Moreover, the fragmentation course occurred in temperature range of 400-600 degrees C and decomposed in 525 degrees C most drastically. The formation of the tar could be regarded as random fragmentation of linear chain segments among ladder structures accompanying with secondary decomposition. After adjusting the middle temperature carbonization technology, depending on the components and formation of tar, the high performance carbon fibers were obtained that had tensile strength of 4.2-4.5 Gpa, elongation at break 1.7%-1.8%, modulus 235-240 Gpa. (c) 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104:1255-1259,2007.