disclosed is a method for measuring a fiber content of polyethyleneterephthalate and polytrimethyleneterephthalate fibers , particularly wherein the method includes : a qualitative analysis step of identifying a presence of polyethyleneterephthalate and polytrimethyleneterephthalate fibers in a mixed - spun fiber product ; and a quantitative analysis step of measuring the fiber content by a microscopic count through difference in color between pet and ptt fibers by birefringence in a cross polarization state of a polarized light microscope . through the present invention , it is possible to simply and accurately calculate the fiber content of pet and ptt fibers in a mixed - spun fiber material in which various kinds of fibers are mixed .