The synthesis of new perovskite TlNiO 3 and a comparative study with TNiO 3 (T=rare earth or yttrium) have been performed. TlNiO 3 was synthesized under high pressure. The domain of pressure and temperature conditions required for obtaining pure TlNiO 3 is very narrow (8GPa=<P, 650=<T=<700 o C). The unit cell volume of TlNiO 3 shows a large deviation from the relationship between effective ionic radii and unit cell volume for TNiO 3 (T=rare earth or yttrium). Temperature dependence of magnetic susceptibility of TlNiO 3 underlines an antiferromagnetic behavior with T N =100K. The structural and magnetic behavior are attributed to the strong covalency of Tl(III)-O bond.