The aim of this study is to demonstrate the influence of the stress parallel to the crack plane on subcritical crack growth in brittle materials by using a numerical code MFPA2D. The mechanism of this influence is also discussed. The curves of subcritical crack extension vs. strain of brittle materials under uniaxial and biaxial stress were obtained through numerical tests with acoustic emission consideration. The results showed that the tensile stress parallel to the crack plane has the effect on crack arrest, while the compressive stress parallel to the crack plane plays important role in crack opening process. The numerical results were consistent with experimental observed result, which shows the reliability of the numerical method, and provides theoretic foundation for failure analysis and life estimation of brittle materials.