Using a plate made of A2017-T6 metal matrix composites reinforced with 10 vol. % and 20 vol. % Al₂O₃ particles, crack propagation rate da/dN of a mode I suface crack by the simultaneous action of plane bending and cyclic torsion is sought. And the effects of crack tip opening stress σ_top, crack opening displacement COD and biaxial stress ratio C on da/dN are examined. When ⊿K is lower than the peculiar level, da/dN decreases with the increase of volume fraction of Al₂O₃ in C=0 and C=-0.55. But da/dN of Al alloy becomes minimum in C=-1 and the effect of Al₂O₃ particles disappears. σ_top increases with Al₂O₃ particles and the decrease of C. On the other hand, COD not always increases with the decrease of C. These phenomena can be explained by the residual compressive stress formed at the surface layer of the specimen by the fatigue test and the surface roughness of crack section.