In the system of PbTe and Sb2Te3, a metastable compound Pb2Sb6Te11 appears by solidification processing. It has been reported that heat treatments decomposes this compound into two immiscible thermoelectric materials forming nanosized lamellar structure. The fraction transformed and the interlamellar spacing was systematically investigated. In this work, the thermal conductivities and the electrical resistivities have been measured as functions of annealing time through the transformation and the coarsening processes to clarify the effect of the fraction transformed and the inter-lamellar spacing. The thermal conductivity of Pb2Sb6Te11 is lower than that after the decomposition. The lattice part of the thermal conductivity of PbTe/Sb2Te3 lamellar samples decreases with deccreasing inter-lamellar spacing. This is considered to be due to the coarsening of the microstructure.