Nanostructured Zn and Ce co-doped In2O3-based ceramics were fabricated using co-precipitation and Spark plasma sintering. It is shown that the nanograins in Zn and Ce co-doped In2O3-based ceramics can lead to 50% reduction in the thermal conductivity of the sample with a 50 similar to nm grain size and meanwhile keep good electric conductivity. As a result, a reported high ZT value of 0.4 at 1050 similar to K was achieved for n-type oxide thermoelectrics. It is expected that the ZT value could reach 0.7 when the grain size is smaller than 20 similar to nm. Our results indicate that nanosized grain is highly desired for reducing thermal conductivity and hence a higher ZT value in oxide thermoelectrics.