The nanocrystalline (Na0.7K0.3)0.5Bi0.5TiO3 plates present room temperature ferromagnetism. The reduction of ferromagnetism for the (Na0.7K0.3)0.5Bi0.5TiO3 plate with subsequent long time and high temperature air-annealing indicates that the observed ferromagnetism is connected with the cation vacancies at/near the surface of nanograins. The results of density functional theory calculation with the local density approximation plus on-site effect method on the magnetism of (Na2/3K1/3)0.5Bi0.5TiO3 supercell show that Na vacancies can introduce a nonzero magnetic moment. In addition, the electric field treatment leads to an enormous enhancement of saturation magnetization for (Na0.7K0.3)0.5Bi0.5TiO3 plates, showing a strong magnetoelectric coupling.