A building block includes two perpendicularly connected square frames, and a hypotenuse frame extended between outer edges of the two square frames. Each of the square and hypotenuse frames has an outer frame portion and an inner cross-shaped rib portion having a thickness twice as large as that of the outer frame portion. The cross-shaped rib portion divides a space enclosed in each frame into four equal divisions, which are four cavities on one of the two square frames; two diagonally opposite round connecting posts and two diagonally opposite cavities on the other square frame; and two diagonally opposite connecting protrusions and two diagonally opposite rectangular cavities on the hypotenuse frame, so that a plurality of the building blocks could be assembled together in different manners to form changeful shapes via engagement of the cavities with corresponding connecting posts or protrusions.