Several rockfalls caused damages on Highway No.2 during an intensive rainfall on August 31, 2013, in Keelung. One of the rockfalls hit a car and the event was captured by a camera. This study analyzes the mechanism of the rockfall based on the video and field investigation. Two major joints which led to the hazard, one was parallel to the coast and the other was perpendicular to the coast. Both of them were almost vertical and cut the rock into several blocks. Weathering processes may have lead to the extension of joints during the heavy rainfall; the infiltration and the surface runoff took the weathered material away, making the rock unable and ultimately leading to the rockfall. The process of this hazard can be divided into topple, roll, fall, bounce and roll. The rockfall took about 23 seconds in total. This paper focuses on the results of field investigation and the mechanism of the rockfall is also discussed.