Single-crystalline KTa0.25Nb0.75O3 (KTN) nanorods with an orthorhombic perovskite structure were synthesized via a polymer-assisted hydrothermal method. The preferred crystallographic orientation of the nanorods were found to be controllable by using different types of polymers in the process; that is, [001]-oriented KTN were obtained upon the use of polyvinyl alcohol (PVA) and [110]-oriented ones were formed with the use of a combination of polyacrylic acid and PVA. The two types of nanorods showed different properties. The [110]-oriented nanorods exhibited a better photocatalytic decolorization efficiency but very likely a lower thermal conductivity (as indicated by the Raman spectrum measurements) than the [001] nanorods.