Polylactide (PLA) is the most important bioplastic on the market due to its good mechanical properties and a permanent growth of the production capacity. But in terms of extrusion foaming the melt strength and extensibility of commercial polylactide is too low. Therefore such commercial grades need to be modified to induce crosslinking, chain extension or grafting by means of reactive extrusion on a twin-screw extruder. In this work different modifiers (chemical as well as physical modifiers) were used. The most effective results were achieved with organic peroxide. With this modifier the molecular weight, the melt viscosity, the strength and the crystallization rate were improved leading to foams with a closed-cell structure and good mechanical properties. Furthermore, the modifier was found to be more effective than a commercial modifier.