MRI-guided microwave ablation for liver tumors has been performed using near-real-time magnetic resonance (MR) images with a double-donut-type open-configuration MR scanner. A microwave coagulator was developed in Japan for hemostasis in hepatic resection under open surgery. Microwave ablation has already been established as one useful minimally invasive procedure for liver tumors. The microwave coagulator operated at 2.45 GHz utilizes dielectric heating similar to microwave ovens. For the ablation of liver tumors, an MR-compatible needle-type electrode was inserted through an MR-compatible puncture needle. The combination of MR image guidance and microwave ablation was quite feasible. During microwave ablation, clear MR images without interference and MR temperature maps could be observed and ablation could be performed without a ground pad. To increase the availability and efficacy of this treatment, various surgical instruments, such as adaptors for the optical tracking system to control image planes, an MR-compatible endoscopic system, navigation software, and a motorized manipulator, have been developed. In this chapter, the outline of this procedure and the advances in peripheral instruments are introduced.