Abstract: A polygonal frame comprising a plurality of framing pieces and the same number of assembly devices. Both ends of each framing piece are slanted so as to fit together with matching slanted framing pieces. The rear side of each framing piece is set with an oblong recess and a longitudinal slot on an appropriate position proximate to each end thereof. Each assembly device comprises: an L-shaped assembly plate which consists of an L-shaped plate portion, two inclined supporting blocks disposed under opposite sides of the plate portion, and an L-shaped supporting rail with each of its two ends connecting to the two opposite supporting blocks; two inclined fastening blocks, each having a lengthwise groove on its lower central portion and a threaded vertical hole to enable a screw to go through; and two screws for engaging with the fastening blocks.