Body centered cubic whiskers having [100] and [110] axes were pulled by use of molecular dynamics using a super computer. The upper yield stress close to the theoretical strength was found. After the upper yield stress, phase transformation was observed at the same time the stress was greatly reduced. A new possible mechanism of twinning is proposed. The whiskers were pulled until they had broken into two pieces. Copper small crystals with and without a crack were sheared. It was observed that the edge dislocations were created at the surface and moved through and escaped from the crystals. Copper small single crystals with crack were pulled. A half dislocation was created near the tip of the crack Sharp yield stress was observed. In medium deformation disclocations on dofferent slip planes were created. Due to the cutting of dislocations the tensile stress increased.