The boron-rich solid of YB 2 5 can incorporate certain amounts of carbon atoms. With increase of carbon content, two kinds of superstructures of YB 2 5 , which modulate the basic structure in the (001) plane, appeared. The framework of boron icosahedra in the superstructures remains the same as that of YB 2 5 and the structural modulation is just caused by the variation of carbon distribution in the voids of the framework. In addition, one superstructure with a large unit cell is quenchable and transformed into another superstructure by thermal annealing at 1400 o C for 80 h.