A series of phenolic resins with different weight-average molecular weights (M-w) and ortho/para (O/P) ratios were prepared. The effect of the phenolic precursor resin structure on the structure and properties of the resulting phenolic fibers was investigated. The structures of the resins and fibers were characterized by nuclear magnetic resonance spectroscopy, gel permeation chromatography, melt rheometry, dynamic mechanical analysis, and thermogravimetric analysis. The results show that the O/P ratio, unsubstituted ortho and para carbon ratio (O-u/P-u), and M-w of the phenolic resins play an important role in determining the properties of the phenolic fibers. The tensile strength of the phenolic fibers increases with increasing novolac precursor O-u/P-u ratios, corresponding to low O/P ratios, at comparable resin M-w values. Also, the tensile strength of the phenolic fibers increases with increasing novolac M-w values at comparable O/P ratios. Phenolic fibers with high tensile strength and good flame resistance characteristics were generated from a phenolic precursor resin, possessing a high weight-average molecular weight and a low O/P value. (C) 2013 Elsevier B.V. All rights reserved.