An improved Single Fiber Tensile Test (SFTT) for the natural fibers was depicted. Natural fibers have irregular shape, and are not uniform along the fiber length and also from one fiber to another. Applying the conventional method, which determine the fiber cross-section by measuring the fiber diameter using optical microscopy, will result in inaccurate properties of the natural fibers with large standard deviation (SD). In the proposed new SFTT method, an accurate cross-section area could be obtained from the Scanning Electron Microscope observation of a flat and clear fractured end surface of carefully selected tensile-tested fibers and calculated using imaging analysis. Applying this new approach, tensile strength of different types of flax fiber, including bast fiber, enzyme-retted and water-retted fiber provided SD of less than 11%, while those of these fibers determined by the conventional approach had SD of over 24%. POLYM. ENG. SCI., 50:819-825, 2010. (C) 2009 Society of Plastics Engineers