A novel and simple hydrothermal synthesis method using metal cyano complex precursors was demonstrated to synthesize nano-sized PrCoO3 and PrFeO3 perovskites. It was possible to achieve significantly improved catalytic properties, but more importantly with thermal stability and purity of perovskite phases in their nanoparticle form. These materials were characterized by means of X-ray powder diffraction, FE-SEM, HR-TEM as well as H2-TPR analysis, and evaluated for their catalytic activity towards CO oxidation reaction. Both the nanoperovskites were able to retain their morphology achieved through hydrothermal treatment, even after high temperature calcination. The catalytic activity of these materials was also compared with conventionally synthesised materials using a solid state method, as well as those reported in the literature. The superior catalytic activity of the PrCoO3 catalyst for CO oxidation over the PrFeO3 catalyst is mainly because of its better redox properties and may also be due to preferential adsorption of CO on PrCoO3 perovskite at low temperature. However, the marked improvement in catalytic activity for both nanoperovskites was primarily due to their improved surface area and better reducibility, which can be attributed to their nanoform. It appears possible to control the particle size as well as the shape of perovskites using this improved synthesis method, while more importantly, it was possible to reduce sintering and agglomeration of perovskites. While the method has potential to apply for synthesis of several other mixed oxide materials in nanoform, it will be important to investigate other physical properties of such thermally stable nanoperovskites.