To study the effect of double filling of the skutterudite iron antimonides on the thermal conductivity, we have synthesized dense samples of Ca 1-xLaxFe4Sb12 (0 ≤ × ≤ 1) and Ca0.3La0.6Fe4-yCOySb 12 (y = 1, 1.5) by the spark plasma sintering technique. Electron-probe microanalysis revealed that the double-filled samples with x = 0.25, 0.5, and 0.75 consist of three phases of micrometer scale; CaFe 4Sb12, Ca-rich phase, and La-rich phase, respectively. The lattice thermal conductivity at 150 K is smaller by 10-20% than that for the end compounds. This is attributable to the scattering of heat conducting phonons by the local fluctuations in both mass and charge of Ca2+ and La3+ ions. In the full range of x from 0 to 1, the thermopower at 300 K stays at 70 μV/K. This value was increased to 120 μV/K by partial substitution of Co for Fe. The dimensionless figure of merit ZT for Ca 0.3La0.6Fe3CoSb12 is estimated to be 0.9 at 800 K, which is comparable to the value reported for p-type Ce 0.44Yb0.32Fe3CoSb12.