Two types of bicrystalline micro pillars (BCMs) involving Σ3 coherent twin boundaries along the pillar axis and single-crystalline micro pillars of two grains constituting the BCMs were fabricated in an oxygen-free copper sample by focused ion beam milling. These pillars were compressed using a nanoindenter to investigate slip transfer across grain boundary (GB). Referring their stress–strain (SS) curves, the possible slip systems were resolved from scanning electron microscope images after deformation and GB interaction criteria with the three largest Schmid factors (SFs) of each grain and the geometric relationship of intersections between the slip planes and the GB. The quite different SS behaviors were observed in the two types of BCMs, of which one was the unstable elastic-almost perfect plastic deformation due to large strain bursts and the other was work-hardening accompanying several small strain bursts. The slip transfer analyses suggest that the slip directions of both grains with the maximum SF are almost parallel to GB plane to the former behavior, and the combination of three slip systems from both grains produces the terraced hardening with repeated slipping across GB and piling-up against GB to the latter.