By using first-principles calculation, we have studied the structure, electronic and magnetic properties of XO (X=Li, K, Rb, Cs) at equilibrium lattice constant in the rocksalt structure. The calculations reveal that the ferromagnetic phase of these compounds is more stable than the nonferromagnetic phase ones and they can be synthetized. All the compounds show half-metallic behaviors at equilibrium lattice constant with an integer magnetic moment of 1μB per formula unit. The half-metallic band gap of these compounds is very large and all the compounds keep their half-metallic characteristic in a wide range of lattice constants. Therefore, we expect that they can be useful in spintronic applications.