This thesis is focussed on the design of a sine wave filter for a GaN-based PWM inverter in order to comply with the EMC requirements according to EN 55011 standard. In addition to the parasitic high-frequency properties of the filter components, also potential sources of interference or coupling mechanisms are analyzed which may impair the filter behavior in the region of higher frequencies. A passive 4th order LC low-pass filter is considered with the aim of achieving low losses while maintaining a high specific power density. For an optimal design of the filter components, toroidal core chokes of various dimensions, core combinations and materials are examined by means of a specifically created calculation software including a material and core database. Thus, feasible and optimal in terms of efficiency and power density choke combinations are generated. The optimized filter inductances are implemented as laboratory samples and tested according their actual properties in the MHz range by means of impedance analyzer measurements. Based on these results a suitable high-frequency equivalent circuit diagram (filter model) is developed and parameterized. The filter implemented actually, however, does not achieve the frequency rejection behavior predicted by lter model as a consequence of cross-coupling effects between the filter stages and also caused by additional noise due to imperfect grounding. Therefore, some modifications such as a special winding technique as well as additional shielding and positioning of specific components have been performed for improving the filter properties. For verification, a GaN half-bridge stage was designed and implemented, which demonstrates that based on the developed optimized and improved filter concept the system complies with the required EMC standard.