The effects of plasma power (1–1.8 kW), oxygen/fuel (0–2.5) and steam/fuel ratios (0–1) on the gasification characteristics of glass fiber-reinforced plastic (GFRP) wastes have been determined in a microwave plasma reactor. GFRP, which is thermosetting plastic composed of glass fibers embedded within a polymer matrix, was used as an experimental sample. While carbon conversion increased with oxygen/fuel ratio, syngas heating value and cold gas efficiency decreased with oxygen supply due to the onset of combustion. With increasing steam/fuel ratio, water-gas shift and ion-reforming reaction favored higher concentration of H2. Increasing the plasma power was found to promote the conversion of carbon dioxide to carbon monoxide. The char surfaces of GFRP that were subjected to variable power and oxygen supplies were analyzed by scanning electron microscopy.