Using COD-R curves, instability of crack growth in ductile material is studied here. Tearing modulus $T_{delta}$, based on the COA concept is defined and the instability criterion $T_{{delta}{app}}$> $T_{{delta}{mat}}$ is compared with experiments. Variations of the COD-R curve and the location of rotation center are experimentally investigated as the initial crack ratio varies. Three-point bend specimens of alloy steel SCM4 are used here and two clip-on gauges are installed in order to obtain the COD-R curve. The results show that the tearing modulus $T_{delta}$ is a good measure to characterize the instability of crack growth. Also it is found that the location of rotation center is almost constant regardless of the initial crack ratio and the amount of the crack growth.