Molecular dynamics simulations are performed to study the plastic deformation behavior of single-crystalline copper film subjected to biaxial tensile strain. The unltrathin film is oriented normal to the 001 crystallographic direction. Beyond a critical value, the film deforms plastically through nucleation, motion and interaction of dislocations. The progress of twinning formed by the motion of Shockley partial dislocations in successive atomic planes is analyzed. Additional stacking faults bounded by twin boundaries are formed in some large newborn twins to release residual strain.