The thermal desorption dynamics of methyl radical from GaAs (100) has been characterized by time-of-flight distribution measurement and resonance-enhanced multiphoton ionization spectroscopy. While the translational energy is nearly thermal neutral, the vibrational distribution in the umbrella mode is superthermal. These results suggest that the methyl radical must desorb promptly from the surface with an "early" transition state. The present study provides a quantitative basis for further theoretical investigations.