Cu with high density nanotwins has been proven to possess excellent mechanical properties and electrical properties. In this study, nanotwins have been successfully introduced in Cu thin films and interconnect lines in trenches down to 150 nm line width. Nanoindentation measurements of hardness show that Cu with some amount of nanotwins has much higher hardness than that of bulk Cu. More interestingly, the hardness at the grain boundary area where there are triple points due to the intersection with some twin boundaries seems to be higher than that inside the grains. Nanotwinned Cu has also been investigated under annealing to study the thermal stability of the nanotwin-modified structure. Copper with nanotwins has slower grain growth than the Cu without nanotwins. Moreover, DC electroplated Cu with very few nanotwins undergoes abnormal grain growth at room temperature while the nanotwinned Cu has very little abnormal grain growth and shows much more uniform grain size. We believe that the effect of nanotwins on the physical properties of Cu is related to the change of the grain boundary microstructure when the twin induced triplepoints are present in the grain boundaries.