Single crystals of new niobium oxides, K2M2Nb4O13 (M = Mg,Fe) were prepared at 1200 degrees C. These niobium oxides crystallize in the K2Ti6O13-type structure and have the monoclinic space group C2/m and Z = 2. The lattice parameters were a = 16.028(6), b = 3.885(3), c = 9.473(3)Angstrom and beta = 99.26(3)degrees for M = Mg, and a = 16.095(2), b = 3.886(2), c = 9.474(1)Angstrom and beta = 99.62(1)degrees for M = Fe. The final R-factors were R = 0.046 and Rw = 0.044 for 2142 unique reflections for M = Mg, and R = 0.026 and Rw = 0.029 for 2144 unique reflections for M = Fe. These niobium oxides are the first example of niobium oxides with the K2Ti6O13-type strcuture. The niobium and magnesium atoms are statistically distributed in triple octahedra sharing edges in K2Mg2Nb4O13, and in contrast the iron atom is located in one of triple octahedra sharing edges in K2Fe2Nb4O13.