This paper concerns a micromechanical study of the tensile strength deterioration of short-glass-fiber reinforced thermoplastics by addition of a slight amount of inorganic agent. Tensile tests were conducted using short E-glass fiber reinforced polyamide 6 with a slight amount of the inorganic addition agents TiO2, ZnO and ZnS. It is found by the tensile tests that the tensile strength decreases with increasing the hardness of the inorganic addition agents, and scarcely depends on the amount of the inorganic addition agents. After measuring the pull-out length of glass fibers on the fracture surfaces of test specimens, the variation of the scale parameter of Weibull moduli is estimated from the cumulative probability of pull-out length. Finally, the tensile strength deterioration is numerically predicted using the data. The predicted values of tensile strength are consistent with the experimental ones.