A block assembly includes at least two blocks detachably connected with each other. Each of the blocks includes a main body having a bottom provided with a receiving space, at least one connecting knob formed on a top of the main body, a plurality of positioning rails formed on a peripheral wall of the receiving space, and at least one positioning piece located in the receiving space. The positioning piece has a periphery provided with at least four arcuate positioning faces. Thus, when the connecting knob of the first one of the blocks is inserted between the positioning piece and the positioning rails of the second one of the blocks, one of the positioning faces of the positioning piece covers and presses the periphery of the connecting knob of the first one of the blocks.