Samples of Sr2-xKxFeMoO6 (x=0, 0.01, 0.02, 0.03, 0.04) are prepared by standard solid-state reaction. The crystal structures and magnetic properties for the ordered double perovskite oxides Sr2-xKxFeMoO6 (0 x 0.04) are investigated. X-ray powder diffraction studies reveal that all the samples are of single phase and each of them has a I4/m symmetry. The anti-site defects in double perovskite oxides of Sr2FeMoO6 may be adjusted by alkali metal element of K doping. The unit cell magnetizations at 280 K are 1.12B for x=0.00 and 1.26B for x=0.04. The cation-ordering and the variation of structure parameters play improtant roles in determining the magnetism in the doping system.