In this study, the effects of the computational spanwise domain length on the flowfield with massive separation and on the flowfield with dynamic stall are investigated by large-eddy simulation. The objective airfoil is NACA0012 and the chord-based Reynolds number is of 2.56 × 105. The objective flowfields are that around a fixed angle of attack of 10 and 25 degrees, and that around a pitching airfoil between AoA of 5 degrees and 25 degrees. The spanwise length effect become significant after the stall, as observed as the attenuation of the large vortices. Observations of the flowfield clarified that the undulation of two large vortices from the leading edge and the trailing edge is one of the mechanisms for the spanwise length effects. The qualitative analysis for this mechanism is performed to address the sufficient spanwise length, and the spanwise length have to be at least 1.0c for the flowfield with large vortex structures so as to resolve its spanwise distribution.