Perfect Archimedes spiral cracks at micrometer scales are found in a colloid film of several hundred nanometers in thickness coated on a glass substrate.To explain the formation of this kind of spiral crack,the relation between the geometrical properties of the Archimedes spiral and the growth kinematics of the spiral cracks are analyzed.Meanwhile,the initiation and propagation of the spiral cracks are speculated upon logically from the specific material system based on sufficient evidence.In addition,we introduce a new cracking phenomenon and provide an insight into the new mechanism for crack propagation in soft material nano films coated on stiff substrates.