A novel type of tunable isolator was presented, which was based on a polycrystalline yttrium iron garnet (YIG) slab loaded on a planar periodic serrated microstrip transmission line that generated circular rotating magnetic field. The direction of circular polarization inside the YIG slab with a DC magnetic bias perpendicular to the transmission line was proved to be opposite, RHCP for forward and LHCP for backward transmission. The non-reciprocal ferromagnetic resonance absorption leads to over 19 dB isolation and <; 3.5 dB insertion loss at 13.5 GHz with 4 kOe bias magnetic field applied perpendicular to the feed line. Furthermore, the tunable resonant frequency of 4 ~ 13.5 GHz was obtained for the isolator with the tuning magnetic bias field 0.8 kOe ~ 4 kOe. The proposed serrated microstrip isolator prototype should have many applications in RF front and other microwave circuits.