The toy building block has a top board, four sideboards, an open bottom and four holding spaces. The top board includes multiple rectangular protrusions and each protrusion has four side surfaces, four recesses and four positioning posts. The recesses are formed respectively in the side surfaces of the protrusion. Each positioning post is formed between two adjacent recesses of two adjacent side surfaces of the protrusion. Each sideboard includes multiple ribs. Each holding space is formed between two adjacent ribs of two adjacent sideboards. To assemble the toy building blocks, the open bottom of one toy building block is used to cover the protrusions of another toy building block to make the holding spaces of the one toy building block hold the positioning posts of another toy building block. Thus, the toy building blocks are securely assembled against rotation relative to each other.