A transformer for resonant inverter, which utilizes magnetic substances infilled between a primary coil and a secondary coil to compel part of the magnetic lines of force of the primary coil not to cross-link with the secondary coil, which thereby achieves regulating leakage inductance. Furthermore, need to renew an existing iron core and bobbin is eliminated, and surmounts the difficult problem of controlling leakage inductance in original configurational designs, thereby decreasing man-hours, enhancing efficiency, and saving on costs.