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tokens: The sheared arcades were likely to reconnect during the flare, and led to the formation of a flux-rope-like core structure that drove the outward eruption. The structure rose slowly with writhing motion and mass drainage, and finally stopped, and gradually became invisible afterward. The process fits into the scenario of a failed eruption of a helical kinked flux rope (e.g., Fan 2005 ; Guo et al. 2010 ; Török et al. 2010 ; Hassanin Kliem 2016 ). The core structure had a peak velocity of 178 km s −1 , and ceased at a height around 0.24 R Sun . For the successful eruption (eruption2), two sets of sheared arcades above the PIL were also identified before its onset. Their corresponding footpoints brightened during the flare, indicating a reconnection between them. The eruption rapidly evolved into a fast CME (with a peak velocity of 1041 km s −1 ) that propagated into the heliosphere. 2. Before both eruptions, continuous flux emergence existed in the source AR. For the source polarity pair, the positive one displayed a faster westward motion than the negative one, resulting in a net convergence toward the PIL. Due to continuous flux emergence and shear motion on the photosphere, magnetic helicity was injected, with a dominating shear term. A larger quantity of magnetic free energy and magnetic helicity was accumulated before the successful eruption. Before the onset of the eruptions, the core region, i.e., the source FPIL of the successful eruption, displayed a larger non-potentiality than the failed eruption. For example, before the successful eruption, more magnetic free energy resided above the FPIL, and larger values were derived for the mean current density, current helicity, and the shear angle that measures the core region’s non-potentiality. The decay index distributions showed no significant difference, although the critical height for the successful eruption was slightly lower (around 3 Mm) than that for the failed eruption. The ratio of magnetic fluxes at 42 Mm and 2 Mm (in the FPIL mask) was smaller before the successful eruption, indicating a relatively weaker confinement. 3. Before the failed eruption, two sets of sheared field lines that corresponded well with the observed sheared arcades, could be identified in the coronal magnetic fields extrapolated by the NLFFF method. After the eruption, the sheared field lines disappeared, while near-potential loops were identified above the PIL. Combined with the observed eruption process, we conjecture that a reconnection between the sheared arcades may have occurred. A similar topology change was found during the successful eruption. Significant enhancements of B h were found in the FPILs after both eruptions, while the value of the Lorentz force change during the successful eruption was an order of magnitude larger than that for the failed eruption. Decreases in magnetic free energy were also found after both eruptions, although the magnitude for the successful eruption was larger