Hydrogenation effects on electrical and optical properties in GaAs epilayers grown on Si substrate by metalorganic chemical vapor deposition have been investigated. In as‐grown GaAs layers on Si substrates, typically four deep levels at 0.81, 0.68, 0.57, and 0.53 eV below the conduction band were observed by deep level transient spectroscopy. After hydrogen plasma exposure at 250 °C for 2.5 h, the 0.68 eV level disappeared. In addition, the reverse leakage current in the hydrogenated sample decreased by three orders of magnitude compared to the untreated sample. These effects persist after dehydrogenation process with a 5 min, 400 °C anneal. The results indicate that the hydrogenation for GaAs‐on‐Si has some benefits to its device application.