In-situ observation of magnetic pulse welding process using a one-turn coil was performed by using a high-speed video camera. High-speed deformation and collision behavior of the metal plates were investigated. The flyer plate traveled toward the parent plate with a high speed by the generated electromagnetic force. A collision velocity of the flyer plate to the parent plate was 250 m/s at the representative welding condition (initial gap distance between two plates: 1.0 mm, discharge energy: 2.5 kJ). It was clearly observed that a part of the flyer plate which was located along the coil bulged toward the parent plate. The collision angle between metal plate surfaces was 0° at the initial collision point, but it increased continuously during the welding. Such a characteristic high-speed oblique collision is considered to result in formation of the wavy interface and gradual changes in its wavelength and amplitude along the welding interface.