A flat wiper blade assembly may be provided that includes: a wiper strip 10 which wipes a wiping surface; a guide spring 30 which supports the wiper strip 10; a first spoiler 41 and a second spoiler 42 which are coupled to the guide spring 30; a seating part 100 which is coupled to the guide spring 30 in an attachable and removable manner; and an adaptor 200 and 200′ which is coupled to the seating part 100. The seating part 100 includes a first fitting portion 110 formed on one end thereof and a first catching portion 120 formed on the other end thereof. The adaptor 200 and 200′ includes a second fitting portion 210 and 210′ which is formed on one end thereof and coupled to the first fitting portion 110 and includes a second catching portion 220 and 220′ which is formed on the other end thereof and coupled to the first catching portion 120.