An amic acid sizing emulsion system was prepared and its properties onto glass fibers were characterized. The emulsion contains small sized particles with a typical core-shell type and has good dispersion stability. The sizing treatment can fill the defects of the unsized glass fiber surfaces and increase the single fiber tensile strength as well as the adhesion between glass fibers and polymer resins. Upon heat treatment, both the sizing film layer and the sized glass fibers exhibit good resistance to high processing temperatures. Moreover, the hygrothermal treatment has also confirmed the good adhesion ability of the sizing as well as its relative good potential to resist the heat and moisture influence in the environment.