A square patch antenna using an in-plane biased yttrium iron garnet (YIG) polycrystalline substrate is presented. The patch antenna is fed through a cross-slot with slots of equal or unequal length to control the polarization. The experimental results reveal that the rotation direction of the elliptical polarization was switched between right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP) controlling the axial ratio continuoulsly without switching devices.The antenna gain was also investigated with respect to this polarization switching and the magnetic bias.