Single fiber fragmentation test (SFFT) was used to investigate the interfacial adhesion in glass fiber-unsaturated polyester composites. A simplified approach was developed for SFFT based on determination of the maximum number of fragments on the fiber at the end of the test. This approach does not involve length measurements and shortens the experiment time to a few minutes. By using a digital camera attached to the microscope, photographs of the coupon were taken during the test, and the number of fragments within the gauge length were counted later. This method allows quick, quantitative comparison of different fibers and matrices. The test samples were prepared by using commercial polyester resin and E-glass fibers having different commercial sizings. SFFT results were in excellent agreement with the macromechanical test done on samples prepared with the same glass fiber and same polyester. The crack modes and debonding phenomena were examined from the microscopic images. Atomic force microscopic (AFM) images of the fiber were examined to get detailed topographic information about fiber surfaces. To improve interfacial adhesion, commercial unsaturated polyester was reacted with 3-aminopropyltriethoxy silane via Michael Addition reaction on the maleate double bonds of the polyester. The resulting silylated polyester was characterized by H-1 NMR spectroscopy. The results of SFFT showed that the maximum numbers of fragments increased 23% on using silylated polyester. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 115: 748-755, 2010