Single pure crystal cooper is usually found to be ductile both in previous simulations and experiments. In this paper, we applied the molecular dynamics simulation to investigate the influence of twinning spacing distance, applied strain rate and model size on fracture behaviour in twinned copper at low temperature. It was observed a brittle-to-ductile transition is correlated with those parameters, when change to below some critical values. The fracture transition process involved the competing mechanism between the homogeneous (or heterogeneous) partial dislocation nucleation close to the crack tip and atoms de-bonding at the crack tip. The twinning and dislocation activities are responsible for the ductile fracture in twinned cooper.